Colts coach Tony Dungy confirmed Monday, after nearly two months of deflecting questions, that Manning was operated on a second time before returning to the field in late August.
Dungy doesn't believe the second operation -- to fix an infected bursa sac -- was the reason the team had a sluggish start this season.
"I don't think it set him back, it was part of the rehab process," Dungy said Monday. "The time frame we were initially looking at was when he came back."
ESPN's Chris Mortensen reported that the second procedure took place 17 days after the original surgery. Doctors were concerned about the fluid that had infected the bursa sac, which was removed in the original operation. So they tacked down all the skin around the knee to prevent that fluid leaking, which could have led to more infection. Manning needed 80 stitches during the second "tack-down" procedure.
Manning was not available in the locker room Monday, a day after the Colts routed the Baltimore Ravens 31-3, in what was easily his best game of the season. Manning typically does interviews only on Wednesdays and after games.
But he told the CBS broadcast team about the second surgery at a pregame meeting, prompting announcers Jim Nantz and Phil Simms to address the issue during Sunday's game.
Manning did not deny having a second surgery, although he had previously referred to it as irrelevant.
The two-time league MVP has said he is continuing to rehabilitate the knee that caused him to miss training camp and all five preseason games and kept him off the field until Aug. 26. Manning wasn't as precise with his passes until Sunday, when he threw for 271 yards and three touchdowns and had his highest quarterback rating of the season (134.7). Earlier this season, Manning acknowledged the knee also had forced him to make more pitches on running plays than straight handoffs.
After Sunday's game Manning indicated the loss of practice time may have affected his early-season play.
"I guess in some ways, this really is like the first week of the season for me," he said. "All I've ever known is taking every rep at every practice and playing in all the preseason games, so I did miss some time there and I've been working through that."
Mortensen reported that Manning had his knee drained during the offseason, believing the condition could be managed that way. Fortunately, the infection set in early enough -- just after his Manning Passing Camp in July -- for the procedures to take place and allow him to return for the regular season. Had it come later, he would have missed start of season.
Manning hated missing training camp but he truly believes offseason on-the-field work during organized team activities is even more important than camp. The knee is "rock hard" and visibly larger than the other, but he has had constant therapy throughout the process, Mortensen reported.
While Manning's knee is no longer a major concern for the Colts, they are looking for reinforcements at running back.
Pro Bowler Joseph Addai left Sunday's game in the first quarter after hurting his hamstring, and Dungy said Monday he wasn't sure whether Addai would play this weekend at Green Bay. Third-string running back Mike Hart could miss much more time after injuring his right knee in the second quarter.
Indianapolis typically does not provide status reports until later in the week.
Dungy said Hart was scheduled to have an MRI on Monday and his prognosis is worse than Addai's, and that could leave the Colts with only one healthy back, former 1,000-yard runner Dominic Rhodes, who took every carry Sunday after Addai and Hart departed.
"It looks like it could be awhile on Mike," Dungy said. "There's a good chance we will [sign another running back] and Chad Simpson would be the first guy we'd look at for sure."
Simpson is an undrafted rookie who has spent all season on the Colts' practice squad.
Had Rhodes been injured Sunday, Dungy said the Colts would have moved rookie tight end Tom Santi to running back.
And if they need another back, the Colts will likely seek a player such as Clifton Dawson, who has experience in the Colts' system.
"We're not necessarily looking for a more veteran guy, but we are looking for a guy who knows our system, knows our blocking assignments, knows our audibles," Dungy said.
The Colts could also get two starters back Wednesday. Left guard Ryan Lilja and outside linebacker Tyjuan Hagler have both been on the physically unable to perform list since training camp opened in late July. The Colts now have three weeks to activate Lilja and Hagler or they will miss the rest of the season, and Dungy believes both could start practicing this week.
Hagler said doctors cleared him to practice a couple weeks ago and he's eager to get on the field, though he doesn't know how long it will take to crack the starting lineup.
"I'm in shape, but game shape and conditioning shape are two different things," Hagler said. "But that's how you get into game shape, you continue to play in games, so we'll see."
ESPN senior NFL analyst Chris Mortensen and The Associated Press contributed to this report.