Attrition, though not as devastating as last year, has presented an early challenge to the Vikings' plans of rolling out a retooled and improved offense.

There remains 2.5 weeks before the regular season opener, and kinks are still being worked out. There's a chance all 11 starters won't be on the field together until then. Guard Brandon Fusco returned to practice from an undisclosed injury this week and is expected to make his preseason debut Sunday at U.S. Bank Stadium. Just as left tackle Matt Kalil, who has drawn praise from head coach Mike Zimmer this summer, has had to bow out of consecutive practices with an apparent leg injury. Starting quarterback Teddy Bridgewater is still easing a reported sore shoulder into action.

And it's not clear whether Adrian Peterson will take his first exhibition carry in five years, which the Vikings haven't ruled out. That's been the longstanding problem with making preseason judgments on the offense, one routinely without its MVP in preparatory games. Though Zimmer still sees a work in progress, having pointed out improved pass protection while noting a need for more from the running game.

One area remaining under construction is the offensive line, where a starting center has yet to be established and first-year position coach Tony Sparano has asked players to embrace different techniques and new approaches in the scheme.

"I think we're getting better at a lot of the things we're working at," Zimmer said Tuesday. "We've installed a lot of new stuff and we're trying to get precise at that. In the game last week, there was a lot of — six out of seven guys would have the right thing and one guy would miss and mess up the play."

There's a small preseason sample of seven throws to judge Bridgewater's progress. Though an assertiveness to push the ball down the field carried over from camp as he completed a 49-yard touchdown to an open Charles Johnson on Aug. 12 in Cincinnati.

Jerick McKinnon, who figures to see an increased role this season, has struggled through two games to find much space on the ground behind the Vikings starting offensive line. He's gained 29 yards on 14 carries [2.1 per] in two exhibitions, including just 15 yards on nine carries last week in Seattle. There were signs of this last year, when McKinnon, who averages 4.9 yards per carry for his career, gained just 1.6 yards per on 25 attempts in Peterson's place. That's before Peterson led the league in carries and rushing yards, leaving some to wonder what he could've done behind a healthier line.

The 2016 version of the offensive line is healthier and has more potential, though it has yet to be settled. The only unsettled competition falls at center, where John Sullivan and Joe Berger have rotated days with the starters this week. Two starters are new in left guard Alex Boone and right tackle Andre Smith. They've also introduced a rookie blocking specialist in David Morgan at tight end.

"There was this one play where we had a nice hole and the tight end gets knocked into the tackle," Zimmer said. "So it gets cut off. There was two times that the back missed the cut. There was two times where we blocked it the wrong way. It's a work in progress, but there really is a lot of good things going on. Obviously we got to do better than two yards per carry, but it's opening up some things in the passing game as well."

The Vikings will still flow through Peterson, though the coaches seek a more balanced approach after only three NFL offenses ran the ball more and no team threw the ball less last season.

"Really what this is all about is a process," Zimmer said. "There's a lot of things that we're doing right now that we may or may not be doing during the season. Some of it is looking at different skill sets of players. Some of it is not showing what we don't want them to see. Every time we go out we want to look great, but there's an end game that is probably more important."