Norv Turner is out as Vikings offensive coordinator after 2½ years on the job. Vikings head coach Mike Zimmer described it Wednesday as a true resignation and a surprising one at that. But even if we take Zimmer at his word, regardless of how you frame the semantics of Turner's departure, a successful offensive coordinator does not typically leave a 5-2 team in the middle of a season.

So what went wrong here, and who is to blame? It's a complicated question, of course, filled with a lot of chicken-or-egg scenarios. Let's dive right in as we attempt to gain some clarity.

•Personnel-wise, Turner didn't always have a ton with which to work. In 2014, he was breaking in a rookie quarterback (Teddy Bridgewater) while dealing with the on-field consequences of Adrian Peterson missing all but one game because of his legal problems.

Peterson (including playoffs) wound up playing in exactly half — 20 out of 40 — games in which Turner was offensive coordinator. Having arguably your most valuable offensive player available for only 50 percent of the games is hard.

Turner also had to deal with the transition on the fly from Bridgewater to Sam Bradford this year. And in all three of his years here, the Vikings offensive line talent has been questionable, at best.

That said, the job of an offensive coordinator is to maximize the skills of the players he has. Turner seemed inflexible at times, relying on a passing attack that requires a good offensive line to thrive.

Zimmer said Wednesday he gave Turner virtually full autonomy to run that side of the ball. When defensive backs fail, it's not the position coach who takes most of the heat — it's Zimmer, the defensive mastermind. When the offensive line fails — as it has in both pass-blocking and run-blocking this year — the offensive coordinator is on the hook.

•Bridgewater's injury and the trade for Bradford might have signaled the beginning of the end. Zimmer praised Turner for being a great coach and teacher, and by all accounts he had a strong rapport with Bridgewater since both arrived with the Vikings at the same time.

But Bradford has great familiarity with Pat Shurmur, the coach replacing Turner as coordinator. Shurmur was Bradford's OC in St. Louis in 2010 and Philadelphia in 2015. He's arguably in a better position to get more from Bradford than Turner could based on familiarity and scheme. In a quarterback league, that means a lot.

•Zimmer said the Vikings offense under Turner was about more than numbers. That's true, to an extent. But the numbers still do tell a story, and it's not pretty.

The Vikings rank 31st in yards per game this season (293.3) after ranking 29th last season and 27th in 2014. If scoring is more your thing, Minnesota is 31st in estimated offensive points per game this season at 15.7. Last year the Vikings were 21st. Two years ago they were 26th.

Those numbers never will tell the full nuanced story of why the offense struggled, but they do paint a pretty clear picture of underperformance during the entirety of Turner's tenure.

The Vikings can be a playoff team with a mediocre (at best) offense. They showed that last year. But if the Vikings are true Super Bowl contenders, they can't afford to underperform on one side of the ball.