Cassel's season is officially over, as Kansas City placed him on injured reserve thanks to a broken right hand suffered in last Sunday's loss to the Denver Broncos.
The 29-year-old finished just 13-of-28 for a season-low 93 yards but threw a touchdown in the 17-10 setback. He will finish the 2011 campaign with 1,713 yards, 10 TDs and nine interceptions on 160-of-269 passes in nine games.
Cassel spent the first four years of his NFL career with the Patriots, and had to fill in for the remainder of the 2008 campaign after star quarterback Tom Brady went down with a season-ending knee injury in Week 1 against the Chiefs.
He ended that game, a 17-10 New England victory, 13-of-18 for 152 yards and one touchdown pass.
To fill the roster spot, the Chiefs promoted wide receiver Jeremy Horne from the practice squad to the active roster.
Tyler Palko will get the nod under center as the Chiefs (4-5) try to halt a two-game skid against the AFC East leaders.