Harrison's status for the team's regular-season opener against the Denver Broncos on Sept. 9. is uncertain, a team official told ESPN NFL Insider Adam Schefter.
Harrison, a four-time Pro Bowler and the 2008 NFL Defensive Player of the Year, has dealt with an inflamed knee since minicamp this spring and has been on the physically unable to perform list for more than two weeks.
The coach dismissed the idea that Harrison could have undergone the surgery during the spring and simply put it off.
"We address things as they arise," he said. "We felt like the procedure was necessary at this time, so we had it."
Tomlin called it a "minor" procedure but added he's not ready to put a timetable on when Harrison will be cleared to play.
"It shouldn't be long," Tomlin said on when he expects Harrison back. "We'll see when we get him back out here and start the process of working him back."
Harrison sat out the preseason opener against Philadelphia last week, and it was unlikely he was going to play in Sunday's game with Indianapolis even before the surgery. Tomlin said the team deemed the procedure "necessary" but wouldn't call it a setback.
With Harrison out, second-year linebacker Chris Carter will likely start at outside linebacker against the Colts.
Earlier in the week, Harrison's agent, Bill Parise, expressed optimism that the 34-year-old linebacker would make a quick recovery if he did have surgery.
"With the exception of this little knee thing, James is probably healthier now than he's been in three years," Harrison's agent, Bill Parise, told the Beaver County Times on Tuesday. "He's extremely fit. He just has to get this irritation behind his kneecap cleared up. That's what's causing the swelling, and (the Steelers) just want to get rid of that. ... We'll just have to wait (and) see how it plays out."
Meanwhile, Steelers running back Isaac Redman said he will have an MRI on Thursday morning for his groin injury. He is uncertain whether he'll play in Sunday's preseason game against the Colts.
"It hurts a little," Redman said. "It's tough to run right now."
Redman is in his first year as the Steelers' starting running back, filling in for Rashard Mendenhall, who is on the physically unable to perform list.
"He's working in a limited capacity in preparation for the game," Tomlin said. "That's all I got in regard to that. He shouldn't be talking to you about MRIs."
Redman's MRI was news to offensive coordinator Todd Haley.
"You guys have more information than I do right now," Haley said. "We're not trying to put too much on him because he is a little bit nicked up. But the way I understood it, we're going to stay on course with the plan to get him some carries in this game."
Information from ESPN NFL Insider Adam Schefter, ESPN.com's Jamison Hensley and The Associated Press was used in this report.