Cassel was sandwiched by Haloti Ngata and Pernell McPhee on the play.
Cassel will now have to pass multiple concussion tests before he's cleared to return to game action on Sunday, when the Chiefs travel to Tampa Bay to take on the Tampa Bay Buccaneers. Cassel's poor play in recent weeks, coupled with his injury, make him unlikely to play.
Kansas City has a bye week in Week 7, so giving Cassel two weeks off to recover from a head injury is likely the safest thing for Cassel.
Brady Quinn came on in relief of Cassel in the fourth quarter, and completed all three pass attempts for 32 yards. Quinn had at touchdown called back because of an offensive penalty. It seems likely that Quinn will at least draw the start for the Chiefs in Week 6, while Cassel recovers from his injury.