Strained something in his lower back. I'd love to provide you with a link, but I have NO clue where I read it.
EDIT: I read it on the Baseball America Prospect Report; says he tweaked his back on Friday. Does not comment on the severity of the injury.
OF (x3): CarGo, M. Bourn, D. Jennings
UTIL (x2): E. Encarnacion, C. Hart
BN: Cuddyer, C. Ross, J. Montero
SP: Price, Gallardo, T. Hudson, Lester
RP: Chapman, Jansen, Rodney, Putz, Cishek, Bailey