Earlier this week, I asked Vikings coach Mike Zimmer when he became comfortable with Case Keenum as his starting quarterback.

“Four or five weeks ago,” he said.

Long after the Vikings’ most vocal fans had decided that Keenum should be pronounced the starter for this and many seasons to come, Zimmer was just beginning to accept that Keenum would continue to play even as Teddy Bridgewater returned to the active roster.

Fans may see this as disloyalty to Keenum, blind loyalty to Bridgewater or just a coaching version of rudeness. It’s merely an experienced NFL coach acknowledging that when it comes to quarterbacks, we don’t know what we don’t know.

Whatever its management and public-relations problems, the NFL remains the dominant sport in North America. It is a billion-dollar industry prone to hyper-analysis, given the relatively few games and plays that occur every year, and the most intense analysis is focused on quarterbacks, who play the most pivotal position in American sports.

Yet NFL teams constantly make bad decisions on quarterbacks, no matter how many hours of game film they watch.

Sunday at U.S. Bank Stadium, two different kinds of promising quarterbacks will start when the Vikings face the Bears. The Vikings made a shrewd signing of Keenum, who has a chance to lead them to their second 13-victory season in franchise history. The Bears used the second pick in the draft on Mitch Trubisky, who has displayed talent and flaws.

Let me let you in on a secret: Neither team really knows what they have.

Keenum has played exceptionally well. What muddles analysis of him, from the Vikings’ perspective, is that Sam Bradford played exceptionally well in Pat Shurmur’s offense. As well as Keenum has played, perhaps the most impressive performance by a Vikings quarterback this season might be Bradford’s one healthy start, in the season opener.

In 1998, Randall Cunningham gave the Vikings one great regular season. He was benched in mid-October the following season. In 2004, Daunte Culpepper became a star. By 2006, he was out of the Vikings’ plans. Keenum has given the Vikings about 2½ high-quality months, and now Bridgewater is healthy and Bradford is ready to return to practice.

The quarterback hierarchy for the remainder of this season is set. The quarterback hierarchy for the future remains muddled, and probably will be determined by Keenum’s playoff performance and evaluations of Bradford’s knee.

The question the Vikings will be asking in February is whether Keenum is a franchise quarterback in isolation, or just when playing for Shurmur, who is likely to become a head coach this winter.

The Bears face a simpler evaluation. It’s always simpler for bad teams. They will fully invest their hope in Trubisky, whether he earns it on the field or not.

How difficult is evaluating quarterbacks?

Tom Brady was drafted in the sixth round. The players chosen by the Patriots before him: Adrian Klemm, J.R. Redmond, Greg Robinson-Randall, Dave Stachelski, Jeff Marriott and Antwan Harris. If Bill Belichick had any inkling that Brady could be a quality starter, much less a historic figure, he was foolish to wait until the sixth round to take him.

Similarly, the Vikings can take credit for signing Keenum to a one-year deal worth $2 million, but if they had any idea he could be a winning starter, they would have signed him to a longer-term deal. They are as surprised by his success as the rest of us.

Russell Wilson and Joe Montana were drafted in the third round. The Chargers chose Drew Brees in the second round.

As rookies, Peyton Manning threw 28 interceptions, Troy Aikman threw nine touchdown passes and 18 interceptions in 11 games and Brett Favre tried to drink himself out of the league.

The smartest and most diligent talent evaluators in the NFL choose quarterbacks knowing that they are defining their reputations, and they still miss most of the time.

As you watch Keenum and Trubisky on Sunday, remember that their teams’ analysis is ongoing, and that in the eyes of Vikings evaluators, Keenum still has much to prove.


Jim Souhan’s podcast can be heard at MNSPN.com. On Twitter: @SouhanStrib E-mail: jsouhan@startribune.com