Ranking the Rockies: No. 1 Nolan Arenado became a star in 2015

Nolan Arenado easily had the best season of any Rockies player in 2015. He bested his position player compatriots in statistics old and new. He led the team in home runs and RBIs, and Arenado’s adjusted offense was the best on the team. His "eye test" defense was superlative. Every advanced defensive metric agreed with our eyes. In terms of total value, as measured by various Wins Above Replacement models, Arenado was anywhere from almost twice to more than three times as valuable as the Rockies’ second best player in 2015, Carlos González. Nolan Arenado was not just the best player on the Rockies in 2015, but he was also the best player on the team by a substantial margin.

Arenado’s season was so inarguably the best among 2015 Rockies that the discussion about the top ranked member of the team in 2015 has to be about where the season fits in Rockies history.

The season Arenado had is one of the 20 best position player seasons in team history. Let’s start with the dingers and RBIs. Arenado accomplished a rare feat when he crossed the 40 home run threshold. He joined an exclusive club that included just 10 other seasons from six other players (González joined him a bit later in the season). Not only that, but Arenado became the first Rockies player to hit 40 home runs in the humidor era. Before he hit his 40th home run of the season on September 26 at home against the Dodgers, the last player on the Rockies to hit 40 home runs in a season was Todd Helton in 2001.

In addition to posting just the 11th 40 home run season in Rockies history, Arenado put up just the 12th 130 RBI season. By doing so, he joined seven other players. The last one was Matt Holliday in 2007. While RBIs provide little value in predicting how well a player will be going forward, they accurately describe what a player did with runners on base. While that is not the entirety of run creation, it is a part of it. And in that respect, Arenado’s season can be counted among the best in Rockies history.

Arenado had a rare season in terms of home runs and RBIs; when we account for the age at which he accomplished these feats, it was an original season. In 2015, Arenado was 24 years old Prior to him, the youngest player in Rockies history to hit 40 homers in a season was Helton at age 26. Helton and Holliday, both at age 27, were previously the youngest players in Rockies history to drive in more than 130 runners. From this perspective, Arenado’s accomplishments are even more pronounced.

We’ve been ranking the Rockies by Baseball Reference’s WAR model (rWAR), which identifies a single number that accounts for an individual player’s total value. It accounts for offense, defense, and base running. Arenado’s rWAR was 5.7. Arenado became the eighth Rockies position player to post a season of at least 5.5 rWAR. In all, his was the 19th such position player season in team history.

The natural follow-up question to the previous paragraph is, "where does it rank in those 19 seasons?" This is a fair question, but it’s not one that rWAR is equipped to answer with precision. We know that a season with, for instance, 41 home runs is not that different from a 40 home run season in terms of value. But the margin of error for counting home runs is zero: 41 is a larger number than than 40.

That is not the case with WAR models. When calculating WAR, the margin of error can be up to 1.0, which is something easily forgotten when fretting over decimal point differences. Arenado’s 5.7 rWAR in 2015 ranks as the 17th best position player season in Rockies history, but it was essentially the same Holliday’s 5.8 rWAR in 2008, which ranks 16th, and it was not all that different from Troy Tulowitzki’s 6.8 rWAR in 2007, which ranks 7th. If we ignore the small differences and classify the best seasons in Rockies history into tiers, Arenado’s 2015 falls into tier three. He has fine company:

All of that leaves us with some compelling questions. One way to look at Arenado’s 40 homer and 130 RBI season is to bask in the rarity of it and it as a sign of things to come. Another way to look at it is to focus a bit more on the rarity of it and acknowledge that there’s a very good chance Arenado will never reach either of these feats again.

These types of seasons are rare for a reason. Vinny Castilla is the only Rockies player to ever hit 40 home runs in three separate seasons, and he did that from 1996-1998, when the balls were dry and juicy. And because Arenado, along with CarGo, was the first humidor-era Rockies player to hit 40 home runs in a season, he’d also be the first to do so twice. It’s just not clear whether or not he ever will, and history suggests he won’t.

If Arenado’s power decreases a bit without anything else in his offense changing, he risks losing a significant chunk of his value. As I’ve written a couple times this year, the most glaring issue in Arenado’s offensive profile is that he doesn’t take very many walks, which suppresses his on base percentage. Indeed, the fact that Arenado made it onto the rWAR leaderboard above was an act of defiance. Arenado’s 2015 OBP of .323 ranks last among those 19 player seasons, and it’s 23 points below the next lowest, which is the .359 Tulowitzki posted in 2007. If Arenado doesn’t increase his on base percentage, hits about ten fewer home runs, and remains the same defensive player, his overall value will take a hit.

But if Arenado can improve his on base ability, he can end up being as good, or better, than he was in 2015 going forward, even with a decrease in some power. If Arenado had 10 fewer dingers in 2015 and upped his OBP with walks only, he’d need a .284/.349/.510 batting line to have an equivalent offensive season (thanks to Ryan Freemyer for mathing that). That OBP would still just be the 24th best in the National League.

It’s entirely possible for Arenado to make these strides. In 2015, Charlie Blackmon upped his walk rate and on base percentage by not swinging so frequently, especially at first pitches. And pitchers have much more reason to fear Arenado than they do Blackmon, so pitches should be further from the strike zone for Arenado. Because he’s a known free-swinger, pitchers have further incentive to stay away from the middle of the plate.

Whatever happens in 2016 and beyond, Arenado’s 2015 will remain among the best position player seasons in Rockies history. Arenado exhibited his power potential and remained the best defensive third baseman in baseball. It will be remembered as the year he became a star.