Westbrook, 25, on Friday underwent arthroscopic surgery on his right knee, the third time in eight months he's had the knee operated on. He is expected to be out at least until the all-star break, Feb. 14-16.
"Russell has been playing pain free but recently had experienced increased swelling," Thunder General Manager Sam Presti said in a statement.
Presti said doctors' examinations disclosed "an area of concern that had not previously existed" and it was determined arthroscopic surgery was necessary.
"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," Presti said.
Westbrook, in his sixth NBA season, tore the meniscus in his right knee in a collision during Game 2 of the Western Conference playoff quarterfinals last season. He had surgery for that injury, but a loose stitch caused swelling and led to another procedure about the time training camp started.
Westbrook missed the first two regular-season games but in 25 games since averaged 21.3 points, seven assists and six rebounds a game. The points-per-game average is second highest for the Thunder while his assist average represents a team high.