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Aaron Boone has big shoes to fill with the Yankees bullpen

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After a tenure of spectacular bullpen management, Boone will need to prove himself.

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Managers are worth wins, just like players. They likely do not produce the same amount of wins as an All-Star caliber player would, but there is some research to show that the difference between the best possible coach and the worst possible manager is about four wins.

That only counts on-field decision making, but we know for a fact that off-the-field decisions matter, too. How you motivate your players, the type of environment you foster, and the quality of practice and coaching does have a non-trivial effect on outcomes despite the fact we can’t measure those things.

This is why despite the fact we know nothing about new Yankees manager Aaron Boone’s on-field strategies, we know he has outlined a strategy for his attitude with the players:

Yet, on the field, as I said, those decisions matter. And from what we know statistically, he has some big shoes to fill. Joe Girardi has been proven to be one of the best at bullpen management in the modern era. Let’s take a look at my favorite managerial stat, wRM+, created by Rob Arthur and Rian Watt at FiveThirtyEight:

“First, we ranked the relievers on each team in every full season since 20003 from best to worst in deserved run average (DRA), which is Baseball Prospectus’s context-neutral metric for evaluating pitcher performance. We then ranked those same pitchers by the average leverage index — essentially, the importance (and pressure) of the moment — at the point when they first entered the game. Finally, we checked how well each team’s ranking of relievers by leverage index matched its ranking by DRA, a correlation we’re calling a team’s reliever management (RM) score... We’re calling the resulting metric weighted reliever management plus (wRM+), and in the style of other “plus” statistics, it’s been rescaled for ease of interpretability: 100 is average, with numbers above 100 corresponding to the percentage factor by which a manager is better than average (or worse than average, for scores below 100).”

So how does Girardi rank? Here are his wRM+’s dating back to 2008, according to the Baseball Prospectus Annual (which you should pick for a host of other info):

  • 2008: 103.2
  • 2009: 104.1
  • 2010: 103.7
  • 2011: 106
  • 2012: 106.7
  • 2013: 107.9
  • 2014: 106.8
  • 2015: 106.5
  • 2016: 107.4
  • 2017: 102.6

It is notable that his last season was his worst, which I have spoken to as to why that was. That being said, Girardi was an elite bullpen manager, where his reliever-to-leverage usage not only helped the Yankees beat back their abysmal 2013-2016 stretch, but it had an effect on the way the bullpens are used in the sport writ large: now, in the postseason, seeing an Andrew Miller perform “fireman” in any inning is nothing to scoff at. Even as late as 2016, that was seen as a revolutionary tactic.

Boone will have a similar challenge with a similar bullpen. Girardi ran into problems by not identifying the strengths of Chad Green and the weaknesses of Dellin Betances and Aroldis Chapman (when they occurred), and it forced him to make misfires throughout the season. Now, Boone will have Green, Betances, Chapman, Tommy Kahnle, and David Robertson, and with a bullpen that large, fluctuations in talent level, command, and nagging injury could affect the optimal usage in each case, and at this point, we have no idea what to expect.

As I said, these are small but significant matters. A win or two doesn’t mean much if the Yankees blow the doors off of the AL East, but if the Bombers find themselves in a position where they are neck-and-neck with the Red Sox or another wild card team, a wRM+ below 100 could be the crucial difference.