I think the best word to describe the 2015 season of Miguel Castro would be "transformative", considering the major changes the 6'5" righty went through this past year (and really, in the last two years). The newly turned 21 year-old Castro was originally signed by the Blue Jays out of the Dominican Republic in the same international signing class as Jesus Tinoco, 2011, for $180,000 as a 16 year-old skinny arm strength projection play.
Castro began his professional career in the Dominican Summer League, pitching 20 1/3 underwhelming frames in 2012 (4.87 ERA). He was anything but underwhelming in his repeat season in the DSL though, tossing 53 innings of 1.36 ERA, 1.37 FIP, 0.98 WHIP, 12.1 K/9, 2.0 BB/9 over 11 games.
That was certainly enough to earn a stateside cameo in the Gulf Coast League (a complex level league below any Rockies stateside affiliates) and a brief stint as a reliever in the Appalachian League (think GJ Rockies).
That success led to a promotion in 2014 to the short season A Northwest League, where the then-19 year-old Castro was again dominant (2.15 ERA, 1.11 WHIP, 9.5 K/9) in 50 1/3 innings against hitters that were on average 2.4 years younger.
Emboldened by those results, the Blue Jays promoted Castro to low A Lansing (think Asheville), where he made four starts (3.74 ERA, 8.3 K/9, 2.9 BB/9 in 21 2/3 innings). Castro was then promoted again (!) to High A (think Modesto) where he made two appearances down the stretch for Dunedin.
At that point, Castro had already made a meteoric rise through Toronto's system, enough for Kiley McDaniel of Fangraphs to give him a 50 FV and rank him sixth in the system prior to the 2015 season:
Castro signed for a smaller bonus as a projection bet in 2012; he sat in the low 90's with rough off-speed and not a ton of feel, but a clean arm and projectable frame. He's taken off since then with 2014 his breakout season. Castro sat 93-96 mph the entire season and got even stronger as the year wore on, sitting 96-98 mph in a short instructs outing in the fall.
Castro is still young, very long-limbed and a power arm more than a pitcher at this point, so there's still plenty of work to be done but the starter traits are here. Castro's changeup is his best off-speed pitch and it consistently above average, helped by the deception from fastball arm speed and also hitters cheating to catch up with the heater. The question, along with the command, is the breaking ball. Castro's slider is below average in some outing and flashes solid average in others, and it is best at his peak velocity, with the arm speed making the pitch crisper.
The Blue Jays were obviously encouraged by the progress Castro showed in 2014, but few were prepared for Toronto to rocket Castro from basically rookie ball to the majors in less than a year. In fact, Castro began the year as Toronto's closer, hitting triple digits on the radar gun en route to four saves and a 4.58 ERA with 12 strikeouts and six walks in 12 1/3 frames before getting sent down to AAA Buffalo in early May. Castro made 13 appearances (five of which were starts) for Buffalo and three additional relief appearances for High A Dunedin (totaling 24 2/3 innings) before getting dealt to the Rockies in the Troy Tulowitzki trade, whereupon a whirlwind season kicked into a higher gear.
The Rockies assigned Castro directly to AAA Albuquerque purely in a relief role. For the Isotopes, Castro made 11 appearances (all in relief), in which he threw 13 2/3 innings of two run, six hit, seven walk, ten strikeout ball. The Rockies called him up in September and Castro made five appearances for Colorado (5 1/3 innings, six runs on six hits and four walks with six punchouts) before getting shut down with a back issue in mid-September.
In totality, Castro pitched for five different teams across three levels in 2015. He made 45 appearances (40 in relief), in which he threw 56 innings, allowing 24 runs (3.86 ERA) on 53 hits and 30 walks (1.48 WHIP) with 56 strikeouts (9.0 K/9). It's hard to draw too many conclusions from those numbers, beyond the fact that it's darn impressive that a 20 year-old was able to do that against mostly AAA/MLB competition.
Here's what McDaniel had to say about Castro at the time of the trade, regarding his future role:
I'm not sure we've learned much about Castro in 2015 with the Blue Jays peculiar usage of the 20-year-old. He's been used mostly in relief and two or three levels above where he should be for the ideal development as a starter. I could guess he'd have problems throwing strikes against the best hitters in the world at an age where most top prospects are in college or A-Ball. I'll leave him as a 50 FV (projected fourth starter or closer, with solid chance for a notch more or less, due to injury/general young pitcher risk).
Meanwhile, MLB.com ranked Castro 10th in Colorado's system in their most recent update:
Castro has gotten stronger since signing and has added velocity to his fastball as a result. He now throws in the mid 90s and can reach 99 mph with the pitch. He commands his fastball well and creates groundballs thanks to its natural life. His changeup is his best secondary offering, while his slider remains a work in progress.
Castro returned to the rotation briefly following his demotion to Triple-A then returned to a bullpen role. It's now up to the Rockies to determine his long-term role. If he can refine his secondary pitches, he has the potential to be a frontline starter. He also could be a dominant closer.
Castro clearly has a big league arm but the answer to three questions, loosely mentioned above, will determine how valuable he will be for the Rockies: 1. Can he command his offerings effectively against the best hitters in the world? 2. Will he develop his slider into a third usable pitch? 3. Will he start at the big league level or be used as a late inning bullpen piece?
If I had to make a guess at this juncture, I'd lean toward the Rockies keeping Castro in the bullpen, if only because the most innings he has thrown in a professional season is still the 80 1/3 frames he pitched in 2014. Stretching Castro out to handle a starter's workload would probably consume most of 2016 in a AA or AAA environment, whereas he could contribute immediately in a multi-inning relief workhorse for the Rockies in 2016. In either case, the Rockies have an extremely talented 21 year-old that is on the cusp of delivering major league value.
Obviously I'd prefer that Colorado pursue the former path, but Jeff Bridich and company might not be big believers in Castro's ability to go deep into games as a starter. This uncertainty led me to place Castro at number 10 in the system on my personal ballot (he'd rank as high as eighth if he were definitely a starter), but I think Castro's upside makes how the Rockies handle him one of the more intriguing storylines to watch in spring training this year.