Pittsburgh Steelers linebacker James Harrison underwent left knee surgery on Wednesday, clouding his chances of returning in time for the start of the regular season.
Coach Mike Tomlin called the procedure "minor" but wouldn't put a timetable on when Harrison will be ready. The four-time Pro Bowler and 2008 NFL Defensive Player of the Year will remain on the physically unable to perform list indefinitely.
The knee limited Harrison during Organized Team Activities in May but Tomlin dismissed the idea that Harrison could have undergone the surgery during the spring and simply put it off.
"We address things as they arise," Tomlin said. "We felt like the procedure was necessary at this time, so we had it."
The 34-year-old Harrison has been one of the most dominant defensive players in the league over the last half-decade but has dealt with a series of setbacks during the last 18 months. He had surgery in February 2011 to deal with a balky back and missed a month last season when he fractured his right orbital bone in a loss to the Houston Texans.
Defensive end Brett Keisel, who sees Harrison regularly in the team's weight room, doesn't think Harrison will be out for an extended period.
"They said he should be fine," he said.
With Harrison and fellow outside linebacker Jason Worilds on the PUP list, it means second-year linebacker Chris Carter will see plenty of action on Sunday when the Steelers (No. 7 in the AP Pro32) face the Indianapolis Colts.
Carter played sparingly on special teams as a rookie last year but has impressed Tomlin during camp.
"He's a guy that has to take that step in the manner that we challenge all our second-year players," Tomlin said. "So far he's accepted that challenge."
The 23-year-old Carter lacks Harrison's explosiveness but has become an effective run-stopper despite his relative lack of size. At 6-foot-1 and 248 pounds, Carter is often tasked with handling offensive linemen who have a distinct advantage.
"The standard is the standard," Carter said. "But I've got a lot to live up to."
Carter was one of the rookies thrown into the mix with little preparation last summer when the NFL lockout ended. Given an entire off-season to get up to speed, he believes he's ready to take a step forward.
"It was a big transition, a big tornado for me [last year]," Carter said. "Now I feel a lot more settled. No need to stress.
"I've got great players around me."