Vikings starting left guard Brandon Fusco, who on Friday was a full participant in practice but was still going through the NFL’s concussion protocol, has been listed as questionable for Sunday’s game against the San Diego Chargers.

If Fusco is not cleared to play, the Vikings could turn to Jeremiah Sirles, who was acquired three weeks ago in a trade with the Chargers.

“It would be kind of fun to play against the team you started with,” Sirles said.

Sirles signed with the Chargers last spring after going undrafted out of Nebraska. He ended up making the team, starting one game and playing in another.

“If you’re not a starter, you have to prepare like a starter because you never know when your number is going to be called,” Sirles said. “That happened to me last year. [I was a] backup and all of the sudden the guy went down and you kind of get thrown in there.”

Coach Mike Zimmer said Sirles has practiced at three positions on the line since arriving. “He’s a big guy, keeps his feet moving, [is] athletic. He’s got some toughness to him,” Zimmer said.

Sirles lined up at Fusco’s left guard spot Wednesday. Fusco returned to practice Thursday.

Rookie Austin Shepherd and second-year lineman Zac Kerin are the other candidates from the active roster to start if Fusco is unable to play.

Looking for a chance

Rookie wide receiver Stefon Diggs impressed in training camp and in the preseason, but he was a healthy scratch the first two weeks of the season. That’s because the Vikings have had more versatile options available for special teams.

“We like him a lot as a player. It’s just that we can [only] have 46 [active players] on game day,” Zimmer said. “He’ll get an opportunity to play this year, I would anticipate. And I think he’ll do good when he does.”

Being a healthy scratch is a new experience for Diggs, who started as a true freshman at Maryland. But he remains upbeat despite the inactivity.

“You’ve just got to understand your role,” said Diggs, a fifth-round pick. “Each day I come in with a positive attitude trying to get better.”

Kalil impresses

Left tackle Matt Kalil has yet to allow a pass rusher to touch quarterback Teddy Bridgewater this season. He allowed two QB hurries in the season opener, but nothing against the Detroit Lions and defensive end Ziggy Ansah, who in Week 6 last season schooled Kalil for 2½ sacks.

Everson Griffen has gone toe-to-toe with Kalil in practice many times over the years, first when they were both at USC and now with the Vikings. The defensive end believes that in recent weeks Kalil has been looking more like the guy who made the Pro Bowl in 2012.

“If you just go back and watch his tape from now to last year, you’re going to see a big difference. Your tape doesn’t lie. What he’s putting on tape right now, that’s who he is,” Griffen said. “The biggest thing for him to do is [to] keep on doing that.”

Injury update

The only player besides Fusco listed on this week’s final injury report was nose tackle Shamar Stephen, who was a full participant in practice all week despite a knee injury. He is probable for Sunday.

The Chargers ruled out four players, including kick returner Jacoby Jones and guard Johnnie Troutman. Three starters — guard D.J. Fluker, tight end Ladarius Green and cornerback Jason Verrett — are questionable.

Saving money

The NFL did not fine linebacker Anthony Barr for his sideline takedown of Detroit quarterback Matthew Stafford on Sunday. Zimmer said this week that a league official told him that Barr “probably” should not have been flagged.