Cassel was injured in the fourth quarter of last Sunday's loss to Baltimore on a hit by Haloti Ngata. No penalty was called on the play.
People in the stands of Kansas City's Arrowhead Stadium cheered when Cassel was hurt, a reaction that drew much criticism.
Kansas City Coach Romeo Crenel said Thursday that Cassel hadn't been cleared by doctors and so he was turning to Brady Quinn to start at quarterback. Ricky Stanzi was installed as the second-team quarterback.
Sunday will mark Quinn's first start since 2009 when he was with the Cleveland Browns.
Kansas City is 1-4 this season going into the game against the 1-3 Buccaneers.