EDEN PRAIRIE, Minn. -- Running back Adrian Peterson is out of his walking boot and will try to practice Friday, while Matt Cassel will remain the Minnesota Vikings' starting quarterback Sunday against the Philadelphia Eagles, coach Leslie Frazier said Wednesday.

Frazier said he's more optimistic about running back Toby Gerhart playing this week than Peterson, but added that Peterson is a quick healer. Gerhart strained his hamstring on a 41-yard touchdown run last Sunday.

Adrian Peterson, who left last Sunday's game with a mid-foot sprain, will try to practice Friday and vows he can play this weekend. Mitch Stringer/USA TODAY Sports

"He can do things that most can't, in terms of coming back from injuries," Frazier said of Peterson. "We were walking up the stairs a few minutes ago, and I said, 'Hey, what do you think?' He said, 'I'll be ready, Coach. Don't worry about me.' That's Adrian."

The coach said Peterson would have to practice on Friday in order to play Sunday. Peterson has been able to carry weight on his right foot, but hasn't done any jogging. A source familiar with the situation told ESPN.com earlier this week that Peterson would not need surgery, but was unlikely to play in Sunday's game. The source said the Vikings would be able to make a definitive decision once they see Peterson run.

Peterson sustained a mid-foot sprain in the second quarter on Sunday against the Baltimore Ravens, as Arthur Brown tackled him while he tried to stretch for a first down. X-rays were negative on Peterson's foot, an MRI showed no structural damage or risk of a Lisfranc injury, and Peterson said on Monday afternoon he planned to play Sunday.

Gerhart, Frazier said, is "far enough along" that he should be able to do some work on Friday.

"That gives me some optimism, but not enough to where we may not have to go to our third back on Sunday," Frazier said.

Christian Ponder will back up Cassel on Sunday. Frazier wouldn't commit to Josh Freeman playing another game, adding Cassel and Ponder "have done some things along the way that have created a situation where we wanted to be able to see Christian play more, now wanted to see Matt a little bit.

"We've watched some things with Josh at practice and meetings. He's done a lot of good things, but where we are now, we want to be able to take a look at Matt and see how he does in back-to-back starts."