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Crafting a Gameplan for the Rockies Bullpen, Pt. 4: The Lefties

Welcome back the “Crafting a gameplan” series, a in-depth and data-based look at the Colorado Rockies pitching staff. In this edition, we’ll be taking a look at the Rockies bullpen.

This is Pt. 4 of a six-part mini-series about the Rockies bullpen integrated within the ‘Crafting a Gameplan’ series. I will discuss the Rockies bullpen as entity, and I will be covering each pitcher in the ‘pen (as of the time of writing) individually at one point or another, but will group athletes who share similarities in pieces so each one has a particular theme. I will eventually dedicate a full piece of the series to take a look at possible free agent options, and the final article will be a recap. The intended structure of this mini-series is as follows:

Each new entry will feature links to the previous parts, displayed up above. You can find the previous entries of the ‘Crafting a Gameplan’ series here:

Thank you very much for reading.

As of now, we’ve taken a general look at the current bullpen situation and gone over six different pitchers, all of them righties. In this entry, we’re going to go over the two lefties currently in the pen: Lucas Gilbreath and newcomer Brent Suter. The Rockies have struggled to find quality southpaws in the last few years, but both Gilbreath and Suter have a good amount of excellent tools to get batters out. Let’s begin.

Lucas Gilbreath

2022 was Gilbreath’s second big-league season, and both have been quite good. Across 85 23 innings, he has a really good 3.78 ERA with the peripherals to match it. He’s recovering from injury right now, but is expected to be healthy for most of 2023, and the Rockies need him: the Colorado native has been a very reliable reliever over the past two years.


Unconventional fastball shape

Gilbreath’s true outlier quality is the shape of his fastball, which is among the most unique in the majors. It’s a four-seam fastball thrown from an average release point and with roughly average extension and vertical carry, but it has a huge amount of glove side cut for a four-seamer, as you can see here. If you want to get a picture of how much of a hard time batters have had squaring this pitch up, consider the following:

Lucas Gilbreath vs MLB Avg

Player wOBA xwOBA ISO xISO EV (MPH) Whiff% Chase%
Player wOBA xwOBA ISO xISO EV (MPH) Whiff% Chase%
Lucas Gilbreath .314 .321 .087 .097 84.9 26,5% 20,9%
MLB 4SF (93-95 MPH) .345 .347 .194 .200 90.3 22,0% 23,4%

Also, Gilbreath has thrown this pitch almost 70% of the time as a big leaguer. I don’t think you need me to tell you that a four-seam fastball thrown that much, at average velocity, is not supposed to generate these results. Check out that difference in exit velo, which is absolutely massive. In fact, among the 277 pitchers to have at least 100 batted ball events against their four-seam in the past two seasons, Gilbreath ranks second-best in exit velo, a full 1.2 MPH ahead of third place. Combined with his (good) tendency to throw it up and to his glove side, his extreme cut profile also gives Gilbreath’s fastball some degree of reverse splits: his four-seamer has a .334 wOBA against lefties and a .302 wOBA against righties so far in his big league career, as righties both chase it more and hit it softer.

This profile leads me to a conclusion that, while I can’t confirm to perfection (because I don’t have biomechanical data), I’m confident in: Gilbreath is a natural at applying force to the outside of the ball, which is why his four-seam cuts as much as it does. This could also lead us to strongly speculate that Lucas is someone who leans towards natural supination at release, which would in turn change our perspective when it comes to expanding his arsenal (if we need to), opening up certain avenues but closing others. Again, this is purely educated speculation, but I’m mentioning it because I intend for these pieces to be educational on top of analytical. Supination biased guys tend to favor cutting four-seams and good breaking ball spin, but they can also struggle with pitches to the arm side. This would be important for later.

Ability to take spin off a splitter

Now, a disclaimer: Lucas has thrown exactly 21 splitters in well over 80 major league innings, so it’s been a non-factor for him so far. However, I want to remark this pitch for two reasons:

  1. Splitters are one of the ideal Coors pitches. Splitters are notorious for their extremely low spin, the lowest in baseball among relatively common offerings. The average MLB splitter over the past few years has been around 1400 RPM, and as we’ve explained many times before, the less your pitch movement relies on spin, the less of a difference altitude makes in its profile. Nobody else on the current pitching staff throws a splitter, so Gilbreath is unique in that regard,
  2. Gilbreath is good at creating low spin. His splitter is often parked in the 1200 RPM range, about half as much spin as his fastball and slider. I wanted to point this out because that’s not something you see everyday. It doesn’t have enough bite on it (only 13 inches of vertical separation between it and the heater, a bit short of the 20-ish range we aim for), but this is noticeable enough for me to raise an eyebrow at it.


With that outlier fastball, we have a really good strength, and there may be something to unlock with his splitter. Now, for the weak spots:

Walks and chases

Like Jake Bird, Gilbreath's issues with walks aren't just about not throwing strikes, though his zone rate is just a little bit below MLB average. It's that, combined with a chase rate well below average, which generates his high walk rate (13.2% in the majors so far). Issues with generating strikes out of the zone is a problem, and one that you can more or less predict when looking at Gilbreath's pitch mix/profile and pitch usage. Lucas throws a ton of fastballs, baseball's least-chased pitch type, and though his fastball doesn't get chased a lot even when compared to others in its velocity range, this is not the true issue. The problem here, in my eyes, his his other main pitch: the slider.

In a vacuum, you might think that his slider is a fine pitch, and it probably is. Gilbreath typically throws it in the 82-83 MPH range, spins it fine, and generates significant depth on it, averaging close to 43 inches of drop (with gravity). It's a really vertical slider, in fact, with under 5 inches of difference of horizontal movement on average between it and the heater. So what’s the problem?

  • Too much velocity difference? Certainly possible. Gilbreath's slider is roughly 11-12 MPH slower than his curveball, a bigger difference than usual, especially for such a vertical profile. And remember that Lucas is not a hard thrower; the time to react his opponents have is not different from average in theory. The general idea is we would want vertical sliders (gyro or not) to be thrown hard, within 6-10 MPH of the fastball. There are outliers, but for many of the more successful sliders of this profile across the big leagues, this is what you see.
  • Too much movement difference? Also a possibility. In the Gameplan series, we've talked many times about how a big difference in shape can make two pitches stand out too much and be recognizable too early during ball flight, allowing good hitters to identify them. A classic example of this would be a big, slow curveball "popping out of the hand", as the term is used. When you look across the big leagues now and you see successful fastball-slider combinations, a difference of anywhere between 15-25 inches of depth is quite common, and what we would consider the sweet spot for pitch tunneling. Gilbreath's slider is around 27-28 inches of depth away from his fastball, right outside of our sweet spot. When combined with its bigger-than-usual velocity separation, it's not outlandish to say that the lack of chases is the result of a combination of both factors.


I think the main decision to be made here is the direction we take Gilbreath’s slider in, and this time I won’t outline a defined plan, but rather give you two different routes and explain the pros and cons.

  • If we add sweep to his slider... it could make sense for a variety of reasons. First off, the glove side cut of Gilbreath’s fastball makes the horizontal tunnel less of an issue (see: Urías, Julio or Ohtani, Shohei). Second, Gilbreath’s fastball tends to perform a bit better against righties than lefties, so a sweeper would give him a great weapon against lefties. However, this would also mean our slider would be less effective against righties (as we expect pitches with a heavy horizontal profile to be against the opposite hand), and there’s also a possibility that the hypothetical sweeper won’t be thrown hard enough to fix the walk issues we’re currently having. This is just speculation, I’d have to get active feedback from the pitcher himself, but if you’re not throwing a vertical slider hard, it’s unlikely that you’ll be able to sweep it at passable velocity. We would also likely need more confidence in the splitter against righties, because even though Gilbreath’s fastball is good, we don’t need to be throwing it 75% of the time.
  • If we go for a harder slider... it could also make sense for a variety of reasons. If fixing our chase issues is what we’re after, this is the direction we should take. We’re aiming for the 85 MPH and 20-ish inches of drop difference range. Because Lucas already creates pretty solid depth on his breaking ball and seems to favor a more vertical profile (his natural arm slot also favors this), throwing a harder and sharper slider makes all the sense in the world to me. Of course, this is the option I would favor with the data points I have available. It’s entirely possible that Gilbreath could throw a good sweeper, don’t get me wrong, but that would require adding that sweep and throwing more splitters at the same time, and I don’t think Lucas’ formula needs a ton of tweaks to it.

That’s Lucas Gilbreath for you. He has one of the most interesting fastballs on the team, and one that can act as a pretty solid backbone. His slider needs an identity, but that’s the only big change I would make. Aside from that, continuing to get reps with the splitter is another natural step to follow.

Brent Suter

Suter was recently acquired by the Rockies off waivers. The 33-year-old lefty had spent his decade-long pro career in the Brewers organization up to this point and has a 3.51 ERA for his MLB career, including a 3.08 ERA since moving to the bullpen full time in 2019. The Rockies picking him up was a good move.


Not only is Suter a good pitcher, he’s also an extremely interesting pitcher on top of it. You’ll see why in just a second; he might be the most unique hurler on the active roster right now. The fact that he works exceptionally quickly is a plus that I won’t put in here, but wanted to mention nonetheless.

Highly unique fastball shape

You know how I said that Lucas Gilbreath’s four-seamer has natural cut? Brent Suter’s cut on his main pitch — which Savant classifies as a four-seamer — makes Gilbreath’s look flat as a pancake. For all intents and purposes, think of it as a full-on cutter: it has cutter velocity and (mostly) cutter shape. It’s also a pitch that MLB hitters simply haven’t been able to square up despite averaging just 86.4 MPH since Suter came into the league: this fastball has an 85.2 MPH exit velo on average since 2016, ridiculously low for a sub-90 MPH fastball thrown almost 70% of the time for his career. Interestingly enough, Suter has mostly thrown it up in the zone throughout his career, which he really should not stop doing as a Rockie, because the difference in results is significant. Remember that 2.5 feet is what Savant considers the height of the dead middle of the zone:

Brent Suter 4SF (2016-22)

Pitch Height wOBA EV (MPH) K% Run Value
Pitch Height wOBA EV (MPH) K% Run Value
> 2.5 feet .289 83.6 25,6% -19
< 2.5 feet .358 88.1 10,9% 4

They just can’t square it up, and I think Suter’s horizontal release point can also play a part in this. He throws from the first base side of the rubber, and with his fastball’s cut, creates some really awkward horizontal approach angles (HAA). Another thing: like Gilbreath’s fastball, this pitch has pretty neutral platoon splits (.314 wOBA for RHH, .302 for LHH). In other words, and to be brief, this is an incredibly unique pitch: a heater with low spin and spin efficiency, and some of the heaviest seam-shift in the majors for a 4SF. I think it might translate solidly to altitude, and it’s an outlier pitch in almost every way. Big plus, even with the low velocity.

Solid secondaries

Suter’s pitch mix is interesting. His fastball usage is pretty similar to both hands (65-75%), and it’s the secondaries taking up the remaining usage that switch. Against righties, he throws his changeup as his main secondary almost exclusively (career high 33.9% usage vs RHH in 2022), and against lefties it’s the curveball and sinker that take the spotlight instead.

Out of these three, the changeup vs RHH is the best-performing pitch, with a .192 wOBA against since Suter became a full time reliever. Regarding his changeup, he made a significant change (get it?) last season. For most of his career, Suter’s changeup was in the 80-82 MPH range, just a few ticks off his fastball, but in 2022 that pitch averaged 77.2 MPH, gained significant depth, and absolutely destroyed righties: they had a .138 wOBA against it, chased it more than ever, whiffed over half the time they took a swing (!!), and produced just one extra-base hit against it all year. You can see it in action here.

Suter’s curveball is well designed as a complement to his fastball. They’re separated by just about 8 MPH and 20 inches of total depth, which is the perfect happy zone for tunneling. It’s quite interesting because while his fastballs barely peak at 90 MPH, Suter’s sharp curveball averages about 79 MPH, above MLB average and extremely high for such a soft tosser. His seam-shifted sinker, which he uses almost exclusively on the inner third against lefties, has a 78.5 MPH exit velo and a -1.8º launch angle against on average since 2019 despite the low velo. In other words, Suter’s secondaries are all optimized to get the most out of them. Changeups to righties, breaking balls and occasional sinkers to lefties. This is a full, cohesive, four-pitch mix built around a unique fastball.

Elite extension

While Suter sits about 86-88 on his fastball, he also has elite extension down the mound. He’s 6-4, has long limbs, and releases the ball much closer to the plate than the average pitcher. As a result, his fastball plays more in the 88-90 MPH range and, most importantly, his curveball and changeup get close to the 80 MPH barrier. This is a big part of why Suter gets all that weak contact; not only is his stuff cohesive, not only does everything he throws move a lot, but everything plays up from its actual velocity. All add to the deception.


This really is just one thing.

Lack of velocity

Even with his extension, Suter is an extremely soft tosser for 2020’s standards. Low velo hurts a pitcher in many ways, one of the main ones being that hitters have more time to react and, as such, often tend to make better swing decisions. Despite Suter’s cohesive and logical arsenal, batters don’t chase a ton against him, especially noticeable on his curveball. Suter’s many qualities allow us to work around this, but it puts a cap on his ceiling.


In Suter, we have a pitcher with a good, varied repertoire, and a very unique fastball as far as shape goes on top of it all. The main hurdle he has to clear is his low velocity. There won’t be many changes to be made here:

Keep throwing changeups to RHH

After the change he made in 2022, Suter’s changeup might just be his best pitch, and its usage climbed accordingly last season. Just a confirmation: keep throwing it a lot against righties, because they really can’t hit it.

A few more curveballs in the zone

Last year, Suter had some issues with his curveball in the rare instances in which he threw it, generating just four called strikes or whiffs in 46 total pitches, for a dreadful CSW% of 8.7%. Since becoming a full time reliever, Suter has thrown his breaking ball in the zone just 27.8% of the the time, the fourth-lowest rate out of the 308 pitchers to toss at least 250 curveballs since 2019. This makes his curve a pitch he doesn’t generate many good strikes with, and I would suggest a few more in the zone. It’s a fine enough breaking ball for that.

That’s about it, really. Suter is a veteran who comes from an organization well-know for excellent pitching development, and he knows what he’s doing. His pitch mix makes sense already, not much for me to add.


The Rockies have struggled to find solid southpaws in the bullpen for a few years, but I don’t think 2023 will be one of those years. Suter and Gilbreath both have good tools to get batters out, and they present two different styles on top of it. For our next entry, we’ll be going over a few free agent targets I’d take a look at if I were in charge. Until then!

This is a study made through my own research and conclusions. In order for it to be truly complete, it would require things such a biomechanical breakdown and some feedback from the pitcher himself, among others, which is data I don’t have access to. Please take what I present more as a suggestion than a stone-cold fact.

★ ★ ★

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