EDEN PRAIRIE, Minn. — The Minnesota Vikings face another quarterback quandary, with Sam Bradford sidelined against Tampa Bay because of an ailing left knee and Case Keenum set for his second straight start.

Vikings coach Mike Zimmer eliminated the mystery before the weekend this time. He ruled out Bradford, who was not at practice Friday because of his injury from the season opener against New Orleans.

Bradford joined the team on the field in a limited basis Wednesday and again on Thursday, when Zimmer sounded upbeat about his progress. ESPN, citing unidentified sources, reported that Bradford was travelling to seek a second opinion from Dr. James Andrews, the noted orthopedist who performed anterior cruciate ligament reconstructions on the quarterback's knee in 2013 and 2014.

Zimmer declined to discuss the report. He even said Bradford saw more action Thursday than Wednesday, declining to characterize the situation as a setback during a terse meeting with reporters after practice Friday.

"He could be back the following week. I don't know. ... But I tell you I don't have a crystal ball," he said.

Zimmer listed Bradford as questionable to play at Pittsburgh last week before declaring him out before kickoff.

The offence struggled in a 26-9 loss to the Steelers, though the factors in the defeat went well beyond Keenum. Bradford was brilliant at home the week before in a 29-19 victory over the Saints.

"We're just as confident as if Sam was playing," wide receiver Jarius Wright said. "We've been with Case ever since OTAs, so we've gotten our reps with Case. Regardless of who the quarterback is, we still have to do our jobs. Everybody just has to do their jobs, and if we can do that then we can win the game."

Keenum has more mobility than Bradford, but that's about the only upside. Bradford was the first pick in the 2010 draft out of Oklahoma and has 79 career starts. Keenum, who was undrafted out of Houston in 2012, has 25 career starts.

"He had a lot of confidence going into last week. He's played a lot of games in this league, and he's won games in this league," tight end Kyle Rudolph said.

Zimmer said last week that Bradford would not need surgery and his absence would be short term. Bradford declined to divulge the specific nature of the injury, but he said it's new to him. Absences, however, are not.

Bradford severely sprained his left ankle in 2011 and missed six games for the St. Louis Rams. The anterior cruciate ligament tear in 2013 limited him to seven games. The ACL tear in 2014 kept him out for the entire season. He had a concussion and a sprained left shoulder that cost him two games with the Philadelphia Eagles in 2015.

"You can't say enough about his toughness," Wright said. "I don't know what's wrong with him, and I don't know his situation, but for him to be able to go out there and practice during the week says a lot about him."

Unfortunately for the Vikings, the quarterback carousel is a familiar ride, too.

This is the 10th year out of 13 that the Vikings have had to start more than one quarterback in a season, with Brett Favre (2009), Christian Ponder (2012) and Teddy Bridgewater (2015) the only ones who made it through unscathed. In a 35-year span since the 1982 NFL strike, the Vikings have enjoyed such 16-game security in only seven seasons, with Warren Moon (1995) and Daunte Culpepper (2000, 2002 and 2004) also on the list.

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