If you take about thirty seconds to peruse this wonderful search on Twitter, you would think the Yankees are in last place. In fact, they are two games back of first place and are currently in possession of the first wild card spot. That isn't great considering they did have a comfortable first place lead at one point, but a surging Blue Jays crew has been hard to halt. Nonetheless, they are still beating pre-season expectations of being an 80-win team, and that's not nothing.
A big part of this has been the success of the bullpen, buoyed by the performances of Andrew Miller, Dellin Betances, Chasen Shreve, and Justin Wilson, who have pitched to a combined 1.99 ERA in 207.2 IP. Considering their combined 5.5 fWAR, that's essentially like having an ace starter (That number is the same as Jon Lester's 2014 fWAR, for what that's worth).
The key to harnessing a bullpen of this quality is making sure these pitchers have the most important plate appearances. There was once a time--not that long ago, in fact--where the best pitcher in the bullpen would only see the ninth inning, and that is probably the worst way to manage a bullpen. Firstly, it under-utilizes one of your best pitchers. Secondly, it ignores leverage. Leverage, for those unfamiliar, is an index that measures how much a plate appearance affects the probability of one team winning a game. This is described in one of Fangraphs' glossary pages:
To calculate it, you are measuring the swing of the possible change in win expectancy... You take the current base-out state, inning, and score and you find the possible changes in Win Expectancy that could occur during this particular plate appearance. Then you multiple those potential changes by the odds of that potential change occurring, add them up, and divide by the average potential swing in WE to get the Leverage Index.
A Leverage Index over 1.5 is considered high leverage, and 1.0 is considered average. It's a bit complicated, but the point to get is that a higher number has a greater impact on the game. If this concept is implemented correctly, then the best pitchers are used in the situations with the highest leverage.
Well, is that case? Using the Baseball-Reference Play Index, I sorted every plate appearance this season by the leverage index, and looked at the fifth inning or later. I chose that inning because, frankly, bullpen decisions earlier than that are pretty limited. Early in a game, a manager is largely forced to keep the starter in in a big spot, otherwise they could blow out the bullpen. So, high leverage late in games is extremely important because pitching decisions are largely at the discretion of Girardi himself.
According to the results, the Yankees (and Girardi) have performed quite well. In 677 applicable plate appearances, opposing hitters have hit .215/.309/.325, and that's good for 7.3 wins by Win Probability Added. But what's the most telling is who pitched in the most important spots. Here are the pitchers in the 25 highest leverage situations:
Of these, there are really only two questionable spots: Esmil Rogers and David Carpenter on April 10th, except for the fact that this was the 19 inning affair. Other than that, either Dellin Betances, Andrew Miller, or Chasen Shreve got the most important spots, and rightfully so. When looking at the whole body of plate appearances, things check out there as well. Betances, Miller, Wilson, and Shreve were used in 51.6% of high leverage situations. In 21.7% of these, the pitcher was either Nathan Eovaldi, Masahiro Tanaka, or Michael Pineda. The one blemish, though, on Girardi's record is his use of CC Sabathia in these situations. In 120 high leverage plate appearances--regardless of inning--Sabathia has an opposing triple slash of .330/.373/.538. Yikes.
Other than leaving Sabathia in his circle of trust, Joe Girardi has managed to maximize his bullpen by largely making sure the best pitchers are pitching in the biggest situations. Traditional role-making would likely assign Shreve, Betances, and Miller to the seventh, eighth, and ninth innings respectively, but flexibility has allowed them to make the most of their abilities. For a team clawing for a playoff spot, every nudge forward helps.