With less than three weeks left before the regular season opener, the Vikings seem to face their biggest questions at linebacker. Yes, it's still likely the trio of Chad Greenway, Erin Henderson and Jasper Brinkley will open as the starters Sept. 9 against Jacksonville in the base defense. But asked Sunday if sixth-year veteran Marvin Mitchell still had a chance to climb up a rung on the depth chart, head coach Leslie Frazier shrugged.

"The door's not closed," Frazier said. "He has an opportunity to compete. We don't want to shut the door yet."

Translation: the Vikings' coaching staff still needs to see more from middle linebacker Jasper Brinkley.

A lot more.

More confidence. More quickness. More decisiveness.

After losing his 2011 season to hip surgery, Brinkley is still cleaning the rust away and has had no problem admitting he's had a long climb back. After a three-week training camp and two preseason games, Brinkley has shown little to prove he'll be the long-term answer at middle linebacker.

"The game is a whole lot faster than practice," Brinkley said after a light practice Sunday afternoon. "I've been away from the game for a year. And each preseason game, I take it and go with it. Because I know it's a learning experience … I have to train my eyes again and [refine] my footwork. That's the first thing you lose when you're away from the game for a year: eyes and footwork."

Acknowledging that need for improvement is a decent first step. But the Vikings are going to need more than self-awareness at middle linebacker if they are going to solidify a defense that allowed 358.2 yards per game in 2011. And if Brinkley can't perform up to expectation, it wouldn't be a total shocker if Henderson moved to play middle linebacker with Mitchell than supplanting him at weakside linebacker.

Henderson and Greenway have been the two linebackers on the Vikings' first-unit nickel defense. And while Greenway thinks Brinkley is steadily showing added comfort in the huddle, he acknowledges that there's plenty of room to grow.

"It's not there yet," Greenway said. "But it's not supposed to be … You need that in-game action. When you see something [on a play], you might see it but not make the play. Because you don't have that reaction back yet to, boom, go and get it. So much of it is getting that burst back, that mode where you're comfortable enough with what you're doing, not questioning your own motives and movements and you just let loose."

After Friday's 36-14 win over Buffalo, Frazier was again open in questioning whether Brinkley was fully engaged from an aggressive standpoint to perform up to potential.

"He has to get his timing down," Frazier said. "He has to get comfortable and he has to believe, 'I'm not going to get hurt. I can play fast and I can play hard.' We want to give him extended reps because of that reason."

Asked to elaborate on that statement Sunday, the Vikings head coach said that over the past four months there have been signs that Brinkley has been tentative, perhaps still conscious of his injury woes.

Brinkley missed portions of the team's organized team activities and the summer mini-camp with groin and hip issues.

"I thought early on, there was some of that [tentativeness]," Frazier said. "I wasn't altogether surprised with all of the time he's missed. When he tried to practice in the OTAs in the offseason, to have trouble then, that will plant a seed in your mind that that could be a guy who's going to struggle with injuries.

"So to see a little bit of that tentativeness early on wasn't a surprise. But the more he's played and the more comfortable and more confident he's become. We have to keep putting him in those situations so he can just turn it loose and not worry about getting injured."