Vikings promote 3rd QB with source saying Sam Bradford a game-time decision

— -- The Minnesota Vikings believe quarterback Sam Bradford will be a game-time decision for Sunday's game at Pittsburgh due to an injured left knee, a source told ESPN's Adam Schefter.

The team plans to work out the veteran quarterback pregame and make a decision then, the source said.

With concern regarding the health of Bradford, the Vikings promoted practice squad quarterback Kyle Sloter to the 53-man roster on Saturday. Bradford is officially listed as questionable for the game.?

A source told ESPN's Chris Mortensen that Bradford underwent an MRI on his knee Tuesday, though the scan did not reveal any specific damage. The quarterback experienced pain, swelling and discomfort in his left knee after the Vikings' season-opening win over the Saints -- the same knee he had surgery on in 2013 and 2014 to repair a torn ACL.

Having three quarterbacks on the active roster foreshadows elevated concern about whether Bradford will be ready to go on Sunday. Vikings coach Mike Zimmer said he "anticipated" that Bradford will start when he spoke with the media on Friday after practice. Bradford is traveling with the team to Pittsburgh and was officially listed as "questionable" on the injury report.

If Bradford is sidelined for Sunday's game, backup Case Keenum will step in as the starter. Keenum started nine of 10 games last season for the Los Angeles Rams and threw for 2,201 yards, nine touchdowns and 11 interceptions.

Sloter has been with the Vikings for only two weeks after spending the preseason with the Denver Broncos.

The Vikings had until 4 p.m. ET Saturday to make additions to the active roster. Minnesota waived safety Antone Exum Jr. to make a spot for Sloter on the 53-man.