The Raiders signed veteran quarterback J.T. O'Sullivan on Thursday and placed starter Bruce Gradkowski on the injured-reserve list, ending his season.
Raiders coach Tom Cable said O'Sullivan will be the No. 3 quarterback for Sunday's game against the San Diego Chargers, behind starter Jason Campbell and Kyle Boller, who was elevated to No. 2.
O'Sullivan, 31, started eight games for the 49ers in 2008, but he has not started any games for any of the eight other NFL teams for which he has played since joining the league in 2002.
"We know a little bit about his background," Cable said. "(Passing game coordinator) Ted Tollner was with him in San Francisco, so he's got some experience and a little bit of the terminology (down)."
O'Sullivan also was with the Chargers this season. The Raiders assigned him No. 13 and used him at practice soon after he arrived and passed his physical. He was not available for comment.
Neither was Gradkowski, who suffered a third-degree separation of his right (throwing) shoulder against the Miami Dolphins on Sunday and isn't expected to be able to throw for a week or two, Cable said. Gradkowski started four games this season.