The Pittsburgh Steelers could be without two key starters for the foreseeable future. Linebacker James Harrison, the 2008 NFL Defensive Player of the Year, had arthroscopic surgery on his left knee Wednesday morning, removing an irritation behind his knee cap that has caused swelling since the spring. RB Isaac Redman is scheduled for an MRI Thursday on his injured groin.

Tomlin described Harrison’s procedure as minor and didn’t have any details on when he would return.

“Shouldn’t be too long,” Tomlin said. “We’ll see when we get him back out here and start the process of working him back.”

Harrison’s knee has bothered him since spring workouts. The five-time Pro Bowler hasn’t practiced since then, including all training camp workouts, and remains on the physically unable to perform (PUP) list. Tomlin didn’t indicate if surgery was an option in May or June. He also didn’t specify whether or not Harrison had a setback or wasn’t responding to treatments.

“We just address things as they arise,” Tomlin said. “We felt like the procedure was necessary at this time, so we had it.”

Harrison, one of the most dominant linebackers in the league the past five years, missed just one game from 2007 to 2010 but has struggled recently with injuries.

During the offseason last year, Harrison underwent two back surgeries in two weeks, with the first to remove part of a herniated disk, and the second to clean up the previous procedure. Last year, he missed four games because of a broken bone in his eye socket.

Harrison’s teammates aren’t worried about the latest procedure.

“He’s a workaholic,” OG Willie Colon said. “Obviously he wants to be ready to go, but you know, sometimes that’s the way it goes. He’ll be fine.”

In the meantime, second-year LB Chris Carter, who has been taking first-team snaps opposite starter LaMarr Woodley, is expected to fill in. LB’s Stevenson Sylvester and Mortty Ivy worked with the second team during Wednesday’s practice.

“He’s represented himself well,” Tomlin said of Carter, adding he experienced productive offseason workouts.

“He’s a second-year player, a guy that has to take that step in the manner of which we challenge all of our second-year players, and thus far, he’s accepting the challenge.”

Carter feels more comfortable this season, and he should with a full year under his belt. It wasn’t like last year’s whirlwind of a season, coming off a lockout when Carter received the playbook two weeks prior to his first preseason game at Washington.

“I’m just willing and able to do whatever the team deems necessary,” Carter said. “The standard is the standard. I have a lot to live up to though.”

So does Redman, who is expected to take over for starting RB Rashard Mendenhall while he recovers from a torn anterior cruciate ligament. Redman revealed his MRI when asked whether or not he would be playing in the team’s second preseason game against Indianapolis on Sunday at Heinz Field.

“We’ll see,” Redman said. “I’m getting an MRI (Thursday) morning.”

Tomlin said Redman is practicing in a limited capacity for Sunday’s game but didn’t deny he would be undergoing the MRI.

“That’s all I’ve got in regards to that,” Tomlin said. “He shouldn’t be talking to you about MRIs.”

Redman took part in individual drills during the beginning of practice Wednesday but wasn’t able to finish, coming up lame during a workout midway through.

“It hurts a little,” Redman said. “It’s tough to run right now. I heal pretty fast, so hopefully I can get past this.”

Redman said the injury happened in training camp.

“It just gradually was sore, and I thought it was a little tight,” Redman said. “I tried to get it stretched, but it just kept getting worse.”

The depth at running back is also thin with third-year RB Jonathan Dwyer battling a shoulder injury and RB John Clay out for the season with a quad injury suffered over the weekend.

Dwyer worked in a limited capacity Wednesday, taking place in individual and 7-on-7 drills. That meant more work for rookie RB Chris Rainey, FB Will Johnson, and RB Baron Batch, who took most of the first-team snaps in practice.

“Obviously one man’s fortune is another man’s opportunity on the practice field and they’ve taken it as such,” Tomlin said.

Follow Steelers RapidReports on Twitter @CBSSportsNFLPIT.