Brady Quinn, who was named as the Kansas City Chiefs starting quarterback earlier this week, suffered a head injury in the first quarter against the Raiders on Sunday and had to leave the game.
He was replaced by Matt Cassel, who had been the starter until he was injured a few weeks ago.
Cassel was deemed healthy enough to play this week, but coach Romeo Crennel named Quinn as his starter anyway.
Quinn was taken to the locker room for further tests after the injury, and it's unclear whether he'll return to Sunday's game.
He had completed 2 of 4 passes for one yard before being injured.