It was from that unsuccessful dive he made in the BOS series. On the broadcast last night they acted like it was pretty bad. They said he couldn't do anything -- throw, bat, even run hard -- without being in tremendous pain. I'll keep my fingers crossed, but if he comes back against TB and is effective I will be very surprised.
Hopefully Rolen will be back and can start to pick up some of the slack, though if Walker keeps this up they shouldn't have too many concerns.