Which Yankees pitchers changed the most in 2017?

Baseball is a game of change. Scoring levels ebb and flow, new strategies emerge as old ones become passe, and sometimes even the ball evolves. Pitchers, batters, fielders, everyone involved with the game is constantly shifting and trying to outmaneuver their opponents.

It goes without saying, then, that the Yankees saw plenty of change in 2017. Breakouts and surprises were the theme of the season, and a bevy of players saw their games change from previous years

It’s worth taking a look at the players that changed the most, beginning with the pitchers. Now, we're not simply looking for the players whose WAR totals changed the most year over year. Instead, it’s more interesting to look for players whose underlying stats, skills, or approach showed very different in 2017.

Masahiro Tanaka

Tanaka has been a fascinating pitcher to follow for his whole career. He has a deep repertoire that he's tinkered with ever since coming over to the big leagues four years ago. The 2017 season was no different, as Tanaka changed his game in a number of ways, even if his results didn't always look better for it.

It's fairly well known at this point that Tanaka doesn't have a great fastball. He's thrown his four-seamer and two-seamer with middling velocity the past few years, with fairly average movement. Hitters have teed off on it, to the tune of a .322 batting average versus the four-seamer and a .296 mark against the two-seamer, per Brooks Baseball.

In 2017, committed more than ever to curtailing his fastball usage. After throwing a cutter, two-seamer, or four-seamer about 45% of the time across his first three seasons, Tanaka used those heaters just 35% of the time in 2017. Tanaka basically began pitching backwards, focusing heavily on his offspeed and breaking pitches, which he used nearly two-thirds of the time. His slider became his most used pitch, turning to it over 30% of the time.

Moreover, using his fastballs less appeared to allow Tanaka to throw them harder than ever when he actually did decide to bring the heat. His velocity was up a full tick, over 93 mph, even sitting in the mid-90s during occasional starts during the summer.

While changing up his approach to focus on his excellent breaking and offspeed offerings and hiding his weaker fastballs seems like a savvy move. It just didn't always work in 2017. Tanaka was dreadful for the first couple of months, and needed an excellent finish to the season to somewhat salvage his numbers on the year. Simply because he was tinkering doesn’t mean he necessarily finding success.

Even so, this change makes sense. Tanaka decided to focus on his best pitches, and allowed himself to throw his fastballs harder than ever. That seems like sound strategy. After finishing the season so brilliantly and deciding to remain in New York, it wouldn't be a surprise to see Tanaka retain the changes to his repertoire and post much better numbers in 2018.

Aroldis Chapman

It may seem weird to list Chapman as having changed more than, say, Luis Severino, or Chad Green. Chapman, on the surface, is just a guy with essentially one strategy and one pitch. He is unambiguous in how he intends to get you out.

Yet the way much of Chapman's season played out was so surprising and so different than the past iterations of himself that he must be among the Yankees' biggest chameleons of 2017. Most specifically, the biggest change Chapman saw was simple: opposing hitters could actually hit him.

The thing that makes Chapman unique, clearly, is his historic velocity. That velocity, in one-inning relief stints, made him essentially unhittable throughout his career. He posted eye-popping swing strike rates and league-low contact rates, because catching up to Chapman when he airs it out for a dozen pitches is an impossible task.

Until this year, that is. Chapmans swinging strike rate plummeted to 13.5%, a career-low, and just a percentage point or two higher than the average for a reliever. His contact rate spiked to 73%, easily a career-high, and, again, barely better than the average for a reliever. In an age when batters are swinging for the fences and less reticent than ever to swing-and-miss, Chapman’s ability to dodge bats regressed heavily.

To put into full context how jarring this development was against the rest of his career, consider these 15-game rolling contact and swinging strike rates for Chapman from 2014 to 2017, courtesy of FanGraphs:

Notice, at least, that Chapman did see a big uptick at the end. He was at his best in October, suggesting that he could have no problem returning to his typical levels next season, just the second year of his record-setting pact with the Yankees.

Even so, such a huge change was mystifying, and it’s not easy to find a simple cause. Chapman’s velocity was down a full mile per hour, according to Brooks Baseball, down from 101 mph to 100 mph. His slider had slightly less horizontal break. It’s possible Chapman’s pitches were just a little less sharp, and that, coupled with worse control, led to hitters putting the ball in play at unprecedented rates.

Regardless of the reason, hitters actually getting wood on the ball against Chapman was a massive shift, and one that almost made him a liability at times during the season. He finished with a flourish, so it’s certainly feasible that this change was more of a blip than a trend that will continue unabated in the coming years. With so much invested in him, the Yankees surely will hope that Chapman can reverse this particular change permanently.