Sam Bradford's sore knee stands between the Vikings' best chances in Pittsburgh or starting the franchise's ninth different starting quarterback since 2011.

The Vikings promoted quarterback Kyle Sloter to the active roster Saturday amid concerns over Bradford's questionable status Sunday against the Steelers. Sloter had been signed to the practice squad earlier this month.

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. The injury limited Bradford in practice throughout the week.

Vikings head coach Mike Zimmer declined to shed light on Bradford's knee injury but said Friday he expected Bradford to play. Even so, Sloter's promotion indicates the Vikings are 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. It'd be a disappointing turn for Bradford after he posted a career-high 143.0 passer rating in the Vikings' season-opening win against the Saints on Monday night.

Sloter was promoted from the practice squad. Safety Antone Exum Jr., a former sixth-round pick with two starts for the Vikings, was waived to make room on the 53-man roster.

The Vikings aggressively pursued Sloter to be the offense's third quarterback earlier this month, giving him nearly active-roster money, $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.