Adrian Peterson vows he can play

— -- 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, who strained his hamstring on a 41-yard touchdown run last Sunday, playing this week than Peterson, but added that Peterson is a quick healer.

Frazier said Peterson told him, " 'don't worry about me, Coach; I'll be ready to go.' "

The coach said Peterson would have to practice on Friday in order to play in Sunday's game as the team needs to see him do something before clearing him to play.

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

A source familiar with the situation told ESPN.com earlier this week that Peterson would not need surgery but he was unlikely to play in Sunday's game.

Christian Ponder will back up Cassel on Sunday. Frazier said he's  unsure if quarterback Josh Freeman will play in another game for the Vikings the rest of the season.

ESPN.com Vikings reporter Ben Goessling contributed to this report.