Westbrook suffered a meniscus tear in his right knee Wednesday during the second quarter of the Thunder's win over Houston in Game 2 of their Western Conference quarterfinal series.
The severity of the injury was not known until Friday.
Although it is possible Westbrook might return during the latter stages of the playoffs, if Oklahoma City can advance, the timing of such a return will not be known until after the operation is performed.
"Our players' health is our first and foremost concern," General Manager Sam Presti said in a statement. "Certainly Russell is a core player for this team, but we are in the midst of the playoffs and I know other players are determined to step up and contribute."
The Thunder hold a 2-0 lead over Houston in the opening-round series, with Game 3 set for Saturday in Houston.
Westbrook, in his fifth NBA season, averaged 23.3 points a game during the regular season. He scored 29 points while playing 37 minutes during the game in which he was injured.
He combined with fellow superstar Kevin Durant to carry Oklahoma City into the NBA Finals last season and the Thunder finished on top of the Western Conference during the regular campaign this year.