 -- The Minnesota Vikings have some concerns about quarterback Sam Bradford's left knee but remain cautiously optimistic about his status for Sunday's game, a source told ESPN on Friday.

Bradford on Tuesday had an MRI on the knee after another source told ESPN that the quarterback had experienced swelling, pain and discomfort following Monday night's victory over the New Orleans Saints.

The source said the MRI did not reveal any specific damage but that scans do not always offer a clear picture in a knee that has undergone multiple ACL repairs.?It is the same knee that underwent ACL repairs in 2013 and 2014, when Bradford was with the Rams.

On Friday, coach Mike Zimmer would not comment on the report.

"I'm not going to talk about who has had MRIs or who hasn't had MRIs," Zimmer said. "Lots of guys have MRIs."

Bradford has been listed as a limited participant on the team's practice reports all week, but the source said the thought was that Bradford would be able to play when the Vikings visit the Pittsburgh Steelers on Sunday.

Against the Steelers defense, Minnesota expects to face intensified pressure with the routine blitz schemes Pittsburgh installs into its game plan. A week after allowing only one sack and keeping Bradford clean throughout the 29-19 win over New Orleans, the Vikings offensive line has an even bigger job this Sunday.

"I'm not going to get too specific, but we're always going to make sure that anybody that has anything that we're trying to protect them," Zimmer said of the protection packages the Vikings would use.

Should Bradford's knee end up holding him out of the game, backup Case Keenum would step in as the starter. Keenum and wide receiver Adam Thielen were seen getting extra work in after practice Friday afternoon. Last season, Keenum started nine of 10 games for the Rams and threw for 2,201 yards, nine touchdowns and 11 interceptions.

Linebacker Anthony Barr, another injury concern for Minnesota, returned to practice Friday in a limited capacity after being out the past two days with a hamstring injury. Barr, Bradford and cornerback Xavier Rhodes (hip) are all listed on the Vikings injury report as questionable.

Zimmer said he anticipates Barr being able to play Sunday.

ESPN's Courtney Cronin contributed to this report.