From the Tigers' Twitter page:
Galarraga left tonight's start with a right ankle contusion. Visit tigers.com for more information.
All things considered, that's not too bad at all. In fact, Roy Oswalt left a start earlier this month with a left ankle contusion, and he went on to make his next start as scheduled. It was a play very similar to the one that forced Galarraga to leave tonight's game, so hopefully his injury will be something that just goes away after a few days.