The Vikings (3-9-1) play host to the NFC East-leading Philadelphia Eagles (8-5) Sunday.
Cassel filled in for Christian Ponder, who has a concussion, in last week's 29-26 loss at Baltimore. He completed 17-of-38 passes for 265 yards and two touchdowns.
In six games this season, three of which have been starts, Cassel has 1,122 yards passing with seven touchdowns and four interceptions.
Peterson, who is second in the NFL with 1,221 rushing yards, left Sunday's game with a right foot injury. He has said he expects to play against the Eagles. Gerhart has a hamstring injury.