Vikings promote QB Kyle Sloter amid concern over Sam Bradford's knee

The Vikings signed quarterback Kyle Sloter to the active roster on Saturday with starter Sam Bradford listed as questionable to play Sunday in Pittsburgh.

Bradford, who boarded the team flight to Pittsburgh on Saturday, is dealing with a left knee injury reportedly requiring an MRI this week that turned up negative. It's the same knee for which Bradford has undergone two surgeries to repair a torn ACL in 2013 and 2014.

Vikings head coach Mike Zimmer declined to shed light on Bradford's injury, but said Friday he expected Bradford to play. Even so, Sloter's promotion indicates they're still concerned Bradford won't suit up vs. the Steelers.

If Bradford doesn't play, backup Case Keenum would get the start. Keenum has 24 career starts, including nine for the Rams last season.

Sloter was promoted from the Vikings practice squad. Safety Antone Exum Jr. was waived to make room on the roster.

The Vikings aggressively pursued Sloter to be the offense's third quarterback earlier this month, giving him nearly active roster money with $20,000 per week. Now on the roster, he'll be bumped up to at least $27,000 per week. Undrafted out of Northern Colorado, Sloter threw for 413 yards and three touchdowns for the Broncos this preseason.

Linebacker Anthony Barr (hamstring) and cornerback Xavier Rhodes (hip) also boarded the team flight to Pittsburgh on Saturday. Both are listed as questionable to play against the Steelers.