The overwhelming majority of commenters have concluded that the ass in question is none other than #65 Phil Hughes.
Here are some factors to take in:
-The ass is white, narrowing it down to a handful of players on the team.
-Tthe uniform in the lockers second digit is a 5
-Mark Teixeira (according to the lady commenters) most likely has a body of a Greek god, so he is ruled out.
-The only other white guy with a jersey containing the number 5 is Phil Hughes.
-The “assman’s” right arm has a tattoo
However, one big issue, brought up by commenter Adam, is that the ass in question could belong to a player standing next to the locker of either Teixeira or Hughes. In that case we may not have seen Phil Hughes ass, but Chad Gaudin instead.
That would be a travesty.