The three-time All-Star had "successful arthroscopic surgery on his right knee" and was not expected to return until after the mid-February All-Star Game break, the Thunder said on their website.
The surgery was the third on the knee this year.
Westbrook has averaged 21.3 points for Oklahoma City this season and his absence is clearly a blow for the team, which has started the season 23-5 - the second best record in the Western Conference.
"Russell has been playing pain free, but recently had experienced increased swelling," team general manager Sam Presti said in a statement.
"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."