With Sam Bradford out, Vikings' playoff aspirations burdened by history of quarterback turbulence

The Vikings will play their second consecutive game without Sam Bradford, who is out for their matchup with the Tampa Bay Buccaneers on Sunday.

A league source confirmed that Bradford was in Florida on Friday to get a second opinion on his knee from Dr. James Andrews, whose office performed operations on Bradford's left knee in 2013 and 2014.

Bradford was limited in practice Wednesday and Thursday, and coach Mike Zimmer said the quarterback did more in practice Thursday than he did Wednesday. When asked if Bradford had a setback after Thursday, however, Zimmer said, "Not really," and later said nothing changed with the quarterback's knee between Thursday and Friday.

Nonetheless, Case Keenum will start again Sunday for the Vikings — the third time in Zimmer's four years as head coach he had to use a second quarterback before the end of September. And if these Vikings are to return to the playoffs, they will have to overcome a quarterback situation that could make it difficult.

Of the six Vikings teams to make the playoffs in this century, only the 2008 team — which won the NFC North at 10-6 while starting Tarvaris Jackson for five games and Gus Frerotte for 11 — reached the postseason while using two different starters. The other five teams — the 2000 and 2004 clubs with Daunte Culpepper, the 2009 team with Brett Favre, the 2012 team with Christian Ponder and the 2015 squad with Teddy Bridgewater — had the same quarterback start all 16 games.

In an era where quarterbacks are more important than ever, losing one — even for a short period of time — can exact a heavy toll.

All six NFC playoff teams last season had a 16-game starter at QB, and 27 of the past 36 teams to make it had a QB play at least 15 games. Keenum still could be that quarterback for the Vikings, in theory, but the path to the postseason for the Vikings appeared at the beginning of the season to be oriented toward Bradford.

Jackson bookended the season as the team's starting quarterback, losing his job after underwhelming performances in two games at the beginning of the season, but regaining it at the end of the season after leading a fourth-quarter comeback in Detroit following Frerotte's exit because of a lower back injury.

Jackson started the Vikings' lone playoff game, completing just 15 of his 35 passes in the loss to the Eagles.

That club had Adrian Peterson winning his first rushing title, by gaining 1,760 yards in 2008, and the league's best run defense. The 2017 Vikings could again be one of the league's best units on the defensive side of the ball, but even with the impressive start of rookie Dalvin Cook, it's difficult to imagine this group having a weapon as dynamic as Peterson.

Zimmer said again Friday he didn't have a crystal ball to predict when Bradford might return, adding it's possible the quarterback could play Oct. 1 against the Detroit Lions, but also possible that he could be out longer.

Bradford said he injured his knee in the season opener against the New Orleans Saints, and a workout before the Vikings' game against the Pittsburgh Steelers last Sunday showed the quarterback unable to do much on the field.

When asked what would tell him if he would be able to go, Bradford said: "I've been through a lot with this knee. ... I've played through quite a few different bumps and bruises with it. I think every time that I've done that, you just know. I don't think there's a checkmark or a benchmark you've got to meet. You've got to be able to play quarterback. You've got to be able to do what the position requires you to do. If you're not able to do those things during practice, it's hard to be able to do them during a game."

And if Bradford isn't able to return to the field in short order, the Vikings' playoff path could be a bumpy one. Recent history isn't on their side when it comes to weathering QB turbulence on the way to the postseason.

Ben Goessling covers the Vikings for the Star Tribune. Twitter: @GoesslingStrib. E-mail: ben.goessling@startribune.com