This was diagnosed as a tight left trapezius muscle, near his pitching shoulder.

WSJ: Pettitte hurt's back as Mariners beat Yankees 3-2

Pettitte, who turns 41 next month, came out of a start against Seattle on May 16 because of a tight left trapezius muscle, near his pitching shoulder.

WSJ: Yanks plan to activate Pettitte on Monday

The impressive outing was big for New York, which placed left-hander Andy Pettitte on the disabled list before the game with a strained left trapezius muscle in his upper back.

WSJ: Blue Jays' Buehrle loses to Yankees again, 5-0