"Russell has been playing pain free but recently had experienced increased swelling," Thunder general manager Sam Presti said in a statement Friday. "After consultation and consideration by his surgeon in Los Angeles, a plan was established to monitor the swelling that included a series of scheduled MRIs.
"On the most recent MRI, it was determined by the surgeon that there was an area of concern that had not previously existed nor was detectable in the previous procedures, and it was necessary to evaluate Russell further. The consulting physician determined that arthroscopic surgery was necessary to address the swelling that was taking place. We know that Russell's work ethic and commitment will help him return to the level of play that we have all come to appreciate."
Westbrook tore the lateral meniscus in his right knee during the second game of the NBA playoffs in April. He underwent an initial surgery for the injury April 27 and had another surgery Oct. 1 to remove a loose stitch from the first procedure.
At the time, the Thunder said he would miss the first four to six weeks of the regular season, but he ended up returning Nov. 3 against the Phoenix Suns, missing just two games.
Westbrook has helped Oklahoma City to a 23-5 record and second place in the Western Conference entering Friday. He is averaging 21.3 points, 7.0 assists and 6.0 rebounds per game in 32.9 minutes.
"He will be missed," coach Scott Brooks said Friday. "Obviously we are a very good team with him, and without him we're still going to be a very good team."
The 25-year-old Westbrook had his seventh career triple-double by the middle of the third quarter in a 123-94 win at New York on Christmas.
Brooks said he didn't know the knee was an issue until after Wednesday's game.
"He's passed every medical benchmark and he was playing great basketball, probably some of the best basketball in the entire league," Brooks said. "We're looking forward to him coming back and getting back to the level that he has played at -- and he will."
The Thunder will be without Westbrook at least through the All-Star Game, which takes place Feb. 15. That means he won't make a fourth straight All-Star appearance and he'll miss at least Oklahoma City's next 27 games.
Thunder still won for the 12th time in 13 games Friday night, beating the Charlotte Bobcats on the road, 89-85.
Reggie Jackson, who came in averaging 12.5 points, started in Westbrook's place, scoring 10 points on 4-of-19 shooting to go with three assists and no turnovers in 30 minutes.
"Guys believe in what he does for our team," Brooks said of Jackson before the game. "He's only going to get better. He's a young point guard in this league."
Brooks said Jeremy Lamb also will see increased playing time. Lamb played 24 minutes at Charlotte, scoring 10 points.
Oklahoma City returns home Sunday to face the Houston Rockets, the same team Westbrook first hurt his knee against when Patrick Beverley banged knees with him while going for a steal after Westbrook had stopped to call a timeout.
Information from The Associated Press was used in this report.