Mike Zimmer keeps saying he doesn’t have a crystal ball. What would help him understand the Vikings’ legacy of quarterback problems is a rearview mirror. One with a crack through the middle.

Vikings fans like to claim they are cursed by big-game losses, but losing in excruciating fashion isn’t a curse, it’s the nature of sport for all but a lucky few franchises.

If they want to claim a curse, they should cite their quarterback history, which features as many hospital gowns as game jerseys.

Sam Bradford’s knee isn’t just sore. It’s the aching juncture of an existential threat to this year’s team and the franchise’s near future.

Since Fran Tarkenton retired, the Vikings have been scrambling like Sir Francis to fill the position.

Some franchises can brag about multiple greats. Joe Montana and Steve Young. Roger Staubach and Troy Aikman. Terry Bradshaw and Ben Roethlisberger.

The Vikings counter with Spergon Wynn and Christian Ponder.

On Monday, Zimmer, the Vikings’ irritable coach, admitted he has no idea when starter Sam Bradford will be ready to play. It could be this week. But if it’s this week, how long will Bradford last on a sore, twice-repaired knee while playing on turf behind an offensive line that again looked leaky in Pittsburgh?

Bradford didn’t even try to run on the soft turf of Heinz Field on Sunday. Six days after his best performance as a pro, he couldn’t even drop back aggressively.

The difference between Bradford and backup Case Keenum is the difference between making the playoffs and throwing away another season.

The Vikings have won one playoff game in the past 13 seasons. And that came with a renegade Packer making a cameo.

The Vikings haven’t won a playoff game with a quarterback they drafted since Daunte Culpepper beat the Packers in the 2004 playoffs.

The past two quarterbacks drafted by the Vikings to play in a Super Bowl: Brad Johnson with the Buccaneers, and Rich Gannon with the Raiders.

Other than Culpepper, the Vikings haven’t won a playoff game with a quarterback they drafted since Wade Wilson beat the Saints in the 1987 playoffs.

And if greatness and Super Bowl championships are the goal, this may be the most damning piece of history of all for the Vikings:

They haven’t drafted a quarterback who would make the Hall of Fame since 1961.

The Vikings’ current roster features two potential franchise quarterbacks, Bradford and Teddy Bridgewater. Both have knee injuries that threaten the team’s season and possibly their careers.

Either could theoretically be the starting quarterback at the end of this season and the beginning of next. But the team has to fear that neither will be able to recover well enough to be the players they are capable of being.

The last true franchise quarterback the Vikings employed, Culpepper, also had his career path ruined by a knee injury.

And their best season with Ponder, their hoped-for franchise quarterback before Bridgewater, ended with him missing a playoff game at Lambeau Field because of an arm injury.

Sunday provided a reminder about the importance of the position. Keenum couldn’t threaten the Steelers downfield, wasn’t particularly good at making blocking scheme adjustments at the line, and allowed an aggressive defense to dictate the flow of the game.

What’s sad about the current situation is that Vikings General Manager Rick Spielman has acquired two good quarterbacks in the past four years.

He landed Bridgewater with the 32nd pick of the first round in 2014, a steal for a player of Bridgewater’s quality.

He spent a first- and fourth-round pick to acquire Bradford, who has completed 72.3 percent of his passes in his 16 games with the Vikings.

Now both have knee injuries that cast doubt on their futures.

Want to feel cursed, Vikings fans?

Ask why the best quarterbacks in recent franchise history seem to have spent as much time in front of orthopedic surgeons as behind center.