The Vikings came away impressed with rookie cornerback Mike Hughes' debut in the preseason opener. Another rookie might not contribute as much this season, but is showing early signs of improvement at a critical position.

Tackle Brian O'Neill, the former Pittsburgh tight end currently in just his fourth year playing offensive line, also received good feedback from coaches, including head coach Mike Zimmer.

"O'Neill played pretty well," Zimmer said. "Some of the things we were working on that he had been doing previously, he did not do, which was good. In the running game he was pretty good, getting to the second level blocking his guys. I thought he improved this week."

It was one exhibition against Broncos backups, and specifically only 47 snaps, but enough for a popular film-grading site like Pro Football Focus to bestow upon him a 'clean' record on 23 pass-blocking snaps. The truth is he made mistakes, like most players.

For instance, O'Neill and running back Mack Brown got their wires crossed on a blitz pickup, leading to a free runner on Kyle Sloter during his nine-yard touchdown throw to Chad Beebe. There are even finer points Vikings coaches are trying to clean up with him ahead of exhibition No. 2 against the Jaguars on Saturday.

"There's a lot to improve on," O'Neill said. "There's a lot of details I need to clean up just like there is at a lot of positions.

"Got a long way to go in both [run and pass blocking]," O'Neill added.

Ideally, the Vikings might want to let O'Neill take a redshirt rookie season, play some special teams and get stronger. He's already added three pounds, weighing in recently at 300.4 pounds, O'Neill said, since the Vikings drafted him. Strength and conditioning coach Mark Uyeyama is focused on building out O'Neill's frame, which was listed as a 6-foot-6-inch, 290-pound tackle at the University of Pittsburgh.

O'Neill said he tries to intake about 6,000 calories per training camp day, which includes nearly three hours of outdoor practices and additional weight lifting sessions.

"That's what I know during camp I need to be around to continue to maintain and gain, and continue to gain muscle mass," O'Neill said. "We're starting to do that in the weight room more, too."

However, O'Neill will still remain busy in the starting lineup. Right tackle Rashod Hill rolled his ankle in practice Monday. Even though Hill returned after being evaluated by athletic trainers, Hill was unable to participate in Tuesday morning's walkthrough, when O'Neill took first-team reps at right tackle and second-team reps at right guard.

The Vikings have also worked O'Neill as a backup left tackle in training camp, preparing him for the likely role of swing tackle this season.