PITTSBURGH -- The Minnesota Vikings will be without quarterback Sam Bradford when they face the Pittsburgh Steelers on Sunday.

Bradford, who has been dealing with pain, swelling and discomfort in his left knee following Minnesota's Week 1 win over the Saints, was one of seven players the Vikings made inactive on Sunday.

Backup Case Keenum will start in place of the injured Bradford. Keenum started his career 0-8 with the Houston Texans in 2013. Since then he has gone 9-7 as a starter.

Bradford went through warmups shortly after 10 a.m. local time where he did not do much to test the strength of his left knee. Bradford did some light throwing and dropped back a few times on passes, but did not run or exhibit much footwork at Heinz Field.

The knee in question is the same knee doctors twice-repaired Bradford's torn ACL in 2013 and again in 2014.

Sources told ESPN earlier in the week that the Vikings were concerned over the health of Bradford's knee but optimistic he would play. There's no clear indication of a play from Week 1 where Bradford may have injured his knee. Minnesota's offensive line kept him clean throughout the game and he was only hit twice.

Keenum and wide receiver Adam Thielen were seen Friday getting extra work in after practice.

Keenum appeared in three games during the preseason where he completed 33 of 48 passes for 330 yards. He didn't throw a touchdown, but he also didn't throw an interception.

Backing up Keenum will be rookie Kyle Sloter, who the Vikings signed to the 53-man roster on Saturday. Sloter spent the preseason with the Denver Broncos before being cut on Sept. 2. Minnesota brought him on as a practice squad quarterback the next day. For the last two weeks he's thrown behind Bradford and Keenum in practice.