Mike Zimmer’s poker face is as convincing as any football coach’s, so he’s never going to reveal a sense of alarm, even if his pulse is redlining like a Porsche.

If he’s privately worried about the state of his offensive line as the Vikings enter Week 1, he refused to walk down that path with reporters on Monday.

That’s understandable because coaches are experts at projecting calm and minimizing glaring trouble spots.

Count me among those with real concerns about Zimmer’s offensive line.

The question mark hanging over that position group continued to grow and intensify as center John Sullivan missed another practice, apparently still recovering from back spasms, an injury that has sidelined the team’s best lineman for three weeks.

Regardless of the public spin at Winter Park, Sullivan’s prolonged absence represents another disconcerting development for a unit that has so much to prove this season.

Even if Sullivan returns to practice this week and plays in Monday’s opener at San Francisco — which still is unknown at this point — the sensitive nature of a back injury creates worry that it could flare up on the 30-year-old again.

The offensive line already is taking on water and the season hasn’t even started.

That’s a red flag flapping in the wind because the success of their season hinges largely on that group’s ability to demonstrate significant improvement.

On paper, the Vikings look like an eight- to 10-win team, possibly a playoff team. But that’s operating under the assumption that their line doesn’t remain a weak link. That’s still to be determined.

For all the justified optimism surrounding Adrian Peterson’s return, Teddy Bridgewater’s development and Mike Wallace’s arrival, the payoff won’t be realized if the line doesn’t perform better than it did last season.

That group holds the key to what the offense can become, not the individual parts of their highly skilled players.

Their line already had areas of concern or unknowns that deserved to be watched closely. Now, they’re dealing with key injuries on top of that.

Right tackle Phil Loadholt is lost for the season because of a leg injury. Sullivan has missed half of camp.

Left tackle Matt Kalil is entering a make-or-break redemption season that will determine his future with the team.

The Vikings also have a rookie at right tackle and a converted tackle playing right guard. And they were so thin in quality depth that they traded for San Diego tackle Jeremiah Sirles when setting their 53-man roster.

So, yeah, there are serious questions and concerns right now.

“I think we’re ready to go,” left guard Brandon Fusco said.

Real games — not preseason auditions for backups — are the only true way to gauge their improvement and determine whether the positive reviews in camp were valid. As Zimmer noted in praising Kalil’s improvement, “I want him to take it to the game this week, so hopefully he will do that.”

Zimmer made protecting Bridgewater a major point of emphasis after the Vikings allowed 51 sacks (fifth most in NFL) and 96 quarterback hits (tied for sixth most) last season.

Sacks aren’t always a direct reflection of a line’s culpability. Quarterbacks hold the ball too long sometimes. Running backs miss blocks. Receivers don’t get open fast enough.

All of those factors contribute. But spotty protection caused Bridgewater to absorb entirely too much punishment as a rookie, and that’s something Zimmer is determined to correct for the sake of his young quarterback’s development and long-term health.

“The sack number last year was way too high,” Fusco said. “That’s something we’re going to take pride in this year, protect the quarterback.”

Asked if he pushed his line harder this camp than previously, Zimmer said, “Much harder.”

Specifically, they devoted more time to pass protection in this training camp. More than anyone can remember. They also did one-on-one pass rushing drills in every single practice.

“That’s something that’s new,” Fusco said. “I’ve never done so many one-on-one reps in my life. It’s not the most fun [thing] to do, but it’s helping us out.”

The Vikings seem confident their extra work will pay dividends, despite all the obvious issues that create skepticism.

The continued absence of their valuable center in practice belongs on that list and provides another stark reminder of the area that could drag the whole thing down if improvement doesn’t materialize.


Chip Scoggins chip.scoggins@startribune.com