Bullpen management is the most difficult part of a modern manager’s job, and that difficulty is only amplified when you manage the New York Yankees, a club with one of the deepest and most dangerous bullpens in baseball history. Every guy “needs work,” but how do you get enough work around for seven guys that would all be closers on a different team?
More to the point, do all guys “need work”? It is often speculated that Aroldis Chapman in particular suffers from rust when he goes three or four or more days without being used in a game. There are a couple of famous examples that stick out, but of course the plural of anecdote is not data. Let’s test it out!
The best way to look at this is, I think, days of rest, which fortunately Baseball Reference provides splits data for. If Chapman is throwing on zero days rest, that means he worked the previous day, one day of rest would be working Wednesday after Monday, etc:
If we look at Chapman’s “second” Yankee career, his two full seasons since signing plus 2019, we can see that there is a relationship between his overall effectiveness and how much time off he gets. K-BB% is, in my opinion, the best measure of a pitcher’s ability, and we can see that it does decline the more rest Chapman gets.
Of course the question is, why? Chapman’s a veteran and a very intense competitor, you’d figure at this stage in his career he wouldn’t have any trouble finding his focus. Is it possible the cause is physical, not mental?
This is from Chapman’s save on Monday, when he was working on one day of rest. He struck out three Orioles with no walks, on 16 pitches—the kind of vintage overpowering save we’re all used to. Look how relaxed his lower half is, how he stays on top of the ball through his whole delivery.
Now look at a pitch from one of his worst performances, a game on three days’ rest where he walked two, gave up two runs, and threw 29 pitches against the Rays:
His mechanics are completely different. His arm flies open, his legs are far stiffer, and surprise surprise, the ball takes off and completes a four pitch walk. This is just one pitch from each appearance, but I think it’s reasonable to conclude that Chapman’s delivery is thrown off by too much rest.
Like a lot of relievers, Chappy’s pitching motion involves a lot of moving parts. It takes a lot to generate the kind of consistent velocity that has made him the game’s hardest thrower for almost a decade, and it sure seems like extended breaks out of action throws a lot of that off. The velocity is still there - even the miss to Tommy Pham is 99 mph, but the command isn’t, and that’s exactly what the chart above reflected.
It’s hard to get everyone in the Yankee bullpen adequate work, but Chapman might be the best pitcher in the bullpen right now. He strikes out the most men, gives up the fewest home runs, and has been worth the most fWAR in 2019 despite ranking just sixth among Yankee relievers in innings. If the Yankees want to get the most out of him, it sure looks like they’ll want to use him with one day rest or so, rather than long stretches that throw off that delivery.