That means backup Tarvaris Jackson likely will start against the Cardinals as the Vikings look to stay a game ahead of Chicago in the NFC North.

Coach Brad Childress says Frerotte is improving and will continue to be evaluated on Saturday and Sunday.

"He's gotten incrementally better," Childress said. "It's not a leaps and bounds thing."

All signs point to Jackson being under center. He took all the snaps in practice this week and played very well in emergency duty last week against Detroit.

In his first action since being benched after Week 2, Jackson was 8-for-10 for 105 yards and a touchdown to rally the Vikings to a 20-16 win over the Lions.

Right tackle Artis Hicks also is doubtful with an elbow injury. Defensive ends Jared Allen and Brian Robison are questionable with knee injuries.

Kick returner Darius Reynaud is out because of a bad foot.

Allen leads the team with 12 1/2 sacks. He's played much of the season with a severely sprained shoulder and said he is hopeful he can play through this injury as well. Allen did not practice on Wednesday or Thursday this week, but did practice on a limited basis on Friday.

Allen said it was up to head athletic trainer Eric Sugarman and Childress to determine if he was ready to play, but he said they will have a hard time convincing him that he is not.

"It's almost as impossible task," Allen said. "But I have a lot of respect for those guys, so I do listen to their opinion. Sometimes you have to get an outside view."