From Footballguys:
KC QB Matt Cassel suffered a sprained MCL in his knee during the team's week three preseason game, and reports out of K.C. indicate that he may be out for up to 4 weeks due to the injury. Head coach Todd Haley refused to update Cassel's status on Monday, August 31st saying 'I'm only going to talk about the players who are on the field.' It appears either Brodie Croyle or Tyler Thigpen will start the season under center for the Chiefs while Cassel recovers - the job will likely fall to whoever performs best in the final preseason game.