Dave Allen of Fangraphs has an interesting claim:
This is the week the Yanks pulled away in the East, and it's all thanks to the bullpen.
In those 5 [close] games the Yankees went 4-1, and based on the scores when they turned to the bullpen (two tied, two down by one, one up by one) that could very easily have been 2-3. So the pen played a huge roll in those wins.
It's hard to dispute the value of the bullpen, though I give a lot of credit to the starters, who have gone 7.2, 7.2, 7, 5, 6, 6, 8, 6 in their last 8 games. In fact, you have to look back to Sergio Mitre's start against Toronto on August 5th to find a starter who went fewer than 5 innings.
So I'd like to argue that what has made the pen so impressive is that they have been asked for few innings (excepting the 15 inning game). The pitchers have been locked into their roles, and the second tier guys (non-Hughes and Mo division) have benefitted from underexposure- only facing the opponent once a series.
For the Fire Joe Girardi crew (and there are/were a couple valid reasons to consider it), I think that he has deftly handled the pen for two years in a row. Since pen (mis)management was the key reason to end the Joe Torre era, Joe G. deserves a lot more credit for mixing and matching so well.