For the first time in more than a month, nose tackle Shamar Stephen strapped on a helmet Tuesday and joined his fellow defensive linemen in individual drills.

While Stephen stayed on the sideline during team drills, it was a positive step for the second-year defender as he works his way back from arthroscopic knee surgery.

Stephen started training camp on the non-football injury list due to a swollen left knee. He began practicing a few days into training camp and played in the preseason opener Aug. 9. The Vikings then shut him down again. He had his knee scoped three weeks ago and wasn't ready to practice until suiting up Tuesday morning.

The timing of the injury was not ideal for a young player who in 2014 surprised by playing 16 games, starting three of them, and making 23 tackles as a seventh-round pick.

"It's never a good time when you have an injury. You're always trying to work back in and play and be with your teammates out there and try to have fun out there," Stephen said. "I wouldn't say it was frustrating, but I'm trying to work on getting everything right and being able to play and help my team."

Stephen is one of four defensive tackles on the roster. That the Vikings decided not to keep a fifth suggests Stephen is close to returning to game action, too. But while he says he feels "pretty good," he declined to speculate on his chances of playing in Monday night's season opener against the San Francisco 49ers.

"That's up to Coach [Mike Zimmer] to decide," Stephen said. "I'm just working my way back in and seeing what happens."

Sullivan still out

Elsewhere on the injury front, center John Sullivan missed his 12th straight open practice because of back spasms. Zimmer had said Monday that he wasn't concerned about Sullivan's injury and that the center would try to practice Tuesday.

But Sullivan, who was last spotted on the Winter Park practice fields on Aug. 18, could not.

Fullback Zach Line, meanwhile, did not participate in practice for a second straight day. He instead did conditioning work on the side with practice-squad linebacker Brian Peters, lagging behind Peters as they both ran from sideline to sideline.

A different look

David Yankey was back in the Vikings locker room Tuesday, sitting at the same locker stall he has occupied since the start of his rookie season. But unlike a week ago, Yankey is no longer on the active roster. He's a practice-squad player now.

Yankey knew he was "kind of on the bubble" heading into last weekend's final cuts. After Saturday's practice, the coaching staff called Yankey upstairs and informed him of his release. He was officially signed to the practice squad on Sunday.

"It's definitely disappointing," he said. "The goal is to ultimately play and reach that level. Before that obviously comes being on the active roster. … But it's definitely a good sign that they still wanted me back for the practice squad."

In three preseason games, Yankey, a fifth-round pick in 2014, allowed 10 pressures in pass protection, according to Pro Football Focus, which graded him as one of the league's worst offensive tackles in pass protection during the preseason.

"Not well enough," Yankey said when asked how he thought he played in the preseason. "I've got to clean up the pass protection for sure and do some work on consistency."

Changing role

After playing sparingly during his rookie year, defensive end Scott Crichton should receive increased responsibility in his second season, not only as a backup to Brian Robison and Everson Griffen, but perhaps also as a part-time defensive tackle.

The Vikings have been tinkering with Crichton as a defensive tackle in pass-rushing situations, giving him a chance to use his quickness against bigger guards.

"He's obviously a big kid," Robison said, "and he's able to get in there and wreak some havoc on those guards because he is a little bit quicker than those guys, so he creates a mismatch there."

While the 2014 third-round pick admits "it's rough down there," he's up for the challenge.

"I don't really care. I just do whatever the coaches tell me to do," he said.