Starting center listed as questionable

James Keefe injured his left shoulder during Friday's practice and is listed as questionable for Sunday night's contest at USC.

The injury is nothing new to the 6-foot-8 senior, who dislocated the same shoulder in a game earlier this season. He injured the same shoulder during the preseason and had surgery to repair a torn labrum in that shoulder in 2007.

So who will guard USC center Alex Stephenson?

Reeves Nelson (concussion) returned to full-contact practice on Thursday and is listed as probable for Sunday's game. But coach Ben Howland had given Keefe the starting nod at center because of his defensive intensity.

Nelson has a tendency to get into foul trouble and it is unclear how he will respond to the concussion he suffered last Saturday against California.