"It's a simple debridement," said Ronnie Barnes, the team's vice president of medical services.
Barnes said Umenyiora's recovery time is expected to be approximately three to four weeks, but a person close to Umenyiora told ESPN NFL Insider Adam Schefter that "it could be four to six weeks or longer."
Giants coach Tom Coughlin did not rule Umenyiora out for the season opener Sept. 11 against Washington. He said it was "no surprise" that Umenyiora needed surgery.
"Hopefully, it'll take less time than the other range and we'll get him back as fast as we can," Coughlin said.
The defensive end just began practicing with the team Monday after he had his knee cleared by team doctors and a specialist in Atlanta last week.
The hope was Umenyiora could manage the injury and perhaps practice every other day, if needed. But Umenyiora experienced swelling in his knee after practicing for three days. On the team's day off Thursday, Umenyiora opted to have surgery.
"It was better to do it now as opposed to midseason," the defensive end said through the team. "It was going to have to be done, the only question was when. If I'm going to miss a little while, I would prefer it be now than at the crucial part of our season. It's the best decision for the team and myself."
Coughlin didn't think anything happened on the field to make the knee worse.
"I don't think anything happened. ... It swelled up. He didn't want to continue like this: practice, swell up, practice, swell up," he said. "So between the doctors and Osi, the decision was made that they would perform this scope and hopefully eliminate that part of it. Allow the recovery time and then allow him to go back to the field without having this as an issue."
Coughlin said that the team has known that surgery might be an option, so "there's no surprise."
The procedure was performed by team physician Dr. Russell Warren at the Hospital for Special Surgery in Manhattan.
"Osi called me at the end of the day yesterday to let me know that he wanted to have the surgery now," Giants general manager Jerry Reese said Friday. "After practicing for three days he had some swelling in the knee, and it was clear that we were going to have to adjust his practice routine to every other day to maintain his knee.
"Osi's feeling is he wants to get it cleaned up now because he is looking forward to playing this season, and he wants to do it without the knee being an issue for him. He told me how much he really enjoyed being back on the field this week, and he's looking forward to getting back out there once he recovers from the procedure."
Umenyiora, 29, missed the first two weeks of practice with a sore knee while also clashing with management over a new contract. The defensive end wanted a raise from the final two years of his deal worth a total of $7.1 million in base salary, or to be traded to a team that would pay him.
The Giants briefly allowed Umenyiora, who had 11½ sacks and forced 10 fumbles last season, to seek a trade through his agent. But after a couple of days, they rescinded permission to work out a deal when no team was willing to offer a first-round pick in return for the pass-rusher.
The Giants offered Umenyiora performance incentives to be added to his contract but the defensive end rejected it. A player can renegotiate his contract only once per season.
Coughlin said Mathias Kiwanuka, who has moved from defensive end to linebacker, will remain at his starting linebacker position. Justin Tuck, who had missed Wednesday's practice with a sore Achilles, is expected to return to practice Friday.
Jason Pierre-Paul, who will start at right defensive end while Umenyiora is out, also is expected to practice Friday after missing a couple of practices this week with back tightness.
Pierre-Paul had two sacks in one quarter in the preseason opener Saturday against Carolina in place of Umenyiora.
Ohm Youngmisuk covers the Giants for ESPNNewYork.com. Mike Mazzeo is a regular contributor to ESPNNewYork.com.