EDEN PRAIRIE, Minn. -- What seemed like a virtual certainty all week is now official: Matt Cassel will start at quarterback for the Minnesota Vikings on Sunday in Baltimore, with Christian Ponder out for the game because of the concussion he sustained last Sunday against the Chicago Bears.

Ponder still has not passed the NFL's concussion protocol, and hasn't been able to practice all week. He worked out on Friday without a helmet, but will still need to clear the concussion test to put himself in the mix to play on Dec. 15 against the Philadelphia Eagles.

For Sunday, Cassel will start, with Josh Freeman backing him up. Ponder is one of three players the Vikings ruled out for the game, along with cornerback Josh Robinson (fractured sternum) and tight end Kyle Rudolph (fractured foot).

Running back Adrian Peterson got some light work on Friday, and will play Sunday with his strained groin. Coach Leslie Frazier said the Vikings will likely have to limit Peterson's practice time the rest of the year because of the injury; they have held him out of Wednesday and Thursday practices for the past three weeks before practicing him on Friday.

"It's similar to what we had to do a year ago, when he had a similar injury [a sports hernia] in the month of December," Frazier said. "We'll have to manage it through the rest of the regular season."

Frazier also said wide receiver Greg Childs practiced for a third straight day on Friday, as he tries to recover from two torn patellar tendons in time to be activated off the physically unable to perform list by next week's deadline and possibly see game action for the first time since he was injured in August 2012. And safety Harrison Smith is still on track to return from injured reserve next week and play against the Eagles after missing two months with turf toe.