Three days before the Pittsburgh Steelers are scheduled to kick off the regular season in Atlanta, they had an unexpected switch at quarterback.

Russell Wilson aggravated the calf injury Thursday that kept him sidelined at the start of training camp and was a limited participant in practice, coach Mike Tomlin said.

Backup Justin Fields worked with the first-team offense during team portions of practice. Kyle Allen was elevated to the backup role while Wilson watched much of the session still wearing his uniform.

“We wanted to exercise some precaution,” Tomlin said. “We’ll see what tomorrow looks like, and we’ll go from there.”

On Friday, the Steelers will hold their final practice before they depart for Atlanta to face the Falcons at 1 p.m. Sunday at Mercedes-Benz Stadium. Wilson was scheduled to speak with reporters after practice Thursday, but he pushed back his media availability slot until Friday.

Wilson originally injured his calf on the eve of the first training camp practice. The injury occurred when he was pushing a sled during a conditioning drill. Wilson didn’t get extended reps until after he sat out the preseason opener against Houston. He started the final two preseason games.

Tomlin officially named Wilson as the Steelers’ starter last week. He didn’t expect the Steelers to have any long-term concern about Wilson’s availability.

Tomlin, though, said he won’t hesitate to turn to Fields on Sunday in the event Wilson’s injury lingers.

“I’m extremely comfortable if that is the case — certainly,” he said.

With Fields running the first-team offense in practice Thursday, he took snaps from rookie Zach Frazier, who will make his first NFL start this weekend.

“Everything went smoothly,” Frazier said. “Justin is a great leader.”

Wilson was given the starting nod because of his previous 12 years of NFL experience that includes two Super Bowl appearances and one championship. The Steelers acquired Fields, who spent his first three seasons in Chicago, to compete for the starting job while also giving the team a mobile threat as a backup.

Both quarterbacks are on one-year contracts.

“Russ is a great competitor and wants to be out there, but if Justin has to step out there, he’s more than prepared to do that,” wide receiver Van Jefferson said. “At the end of the day, we’ll go out and play. No matter who is out there, the job still has to get done.”

Fields started 38 games in his three seasons with the Bears, with 28 of those appearances coming over the past two seasons. He’s also a former 1,000-yard rusher who led the NFL with a 7.1 yards per carry average in 2022.

“We do a lot of things after practice to get our chemistry down with him,” Jefferson said about Fields. “For the time that Russ was out, we saw the way he performs and how great he was out there. I feel like if he’s out there, no one is going to miss a beat.”

Joe Rutter is a TribLive reporter who has covered the Pittsburgh Steelers since the 2016 season. A graduate of Greensburg Salem High School and Point Park, he is in his fifth decade covering sports for the Trib. He can be reached at jrutter@triblive.com.