The team placed Jackson on injured reserve Thursday with a turf toe injury. The Vikings signed veteran quarterback Patrick Ramsey, who hasn't played in a game since 2008, to fill his roster spot.
Ramsey was let go by the Dolphins last month and hasn't started a game since 2005.
Jackson left last Monday's loss against the Giants twice with an injury, the second time because of the turf toe issue. Jackson leaned heavily on a railing to get up steps toward the locker room after the game.
Favre was feeling better Tuesday, according to coach Leslie Frazier, after his NFL-record streak of starting 297 straight regular-season games ended Monday with him on the sideline at Ford Field in Detroit during Minnesota's loss to the Giants.
He is nursing a sprained throwing shoulder and a numb hand, but an ultrasound on Favre's neck and shoulder revealed no nerve damage, Frazier said.
If Favre can't play and the Vikings don't start Ramsey, the only other quarterback on the Vikings' roster is rookie Joe Webb, who was going to be used as a wide receiver until the decision was made to make him a quarterback.
Webb hurt his hamstring on Dec. 5 against the Bills, and he's a sixth-round draft pick whose athletic ability can't mask the fact that he's raw and inexperienced with running the offense.
Information from The Associated Press was used in this report.