The team announced that Westbrook needed the operation after experiencing swelling in his right knee, which he injured during last season's playoffs.
The three-time All-Star had surgery on the same knee in May to repair a torn lateral meniscus.
Thunder general manager Sam Presti said that the swelling in Westbrook's knee was caused by a loose stitch from the previous surgery and that Westbrook's meniscus has healed properly.
Presti commended Westbrook for his hard work in rehabbing from the original surgery but acknowledged that an additional operation provided the 24-year-old with "the best chance" to return at 100 percent.
"His performance levels have been tremendous, and he's looked great in the part of practice that he's been cleared to go through," Presti said. "So we were very, very encouraged in that respect. But when the swelling wasn't subsiding, we made the decision to have the knee evaluated."
Westbrook said last week that once he's fully recovered, he doesn't plan on changing anything about his playing style, including his proclivity for driving to the basket.
Westbrook averaged 23.2 points and 7.4 assists last season for the Thunder, who finished with the top seed in the Western Conference playoffs. He was injured in Game 2 of a first-round series against the Houston Rockets, a matchup won by Oklahoma City in six games.
Westbrook was hurt on April 24 when Rockets rookie Patrick Beverley lunged for a steal as Westbrook stopped to call a timeout in the second quarter. He remained in the game, and it wasn't until two days later that the team said he'd miss the rest of the playoffs.
Beverley has said he wasn't trying to hurt Westbrook and was simply trying to make a play.
In the conference semifinals, the Thunder clearly missed Westbrook's presence, losing to the Memphis Grizzlies in five games.
Some of the offense is expected to be picked up by third-year guard Reggie Jackson, who averaged 13.9 points in the playoffs after averaging just 5.3 during the regular season. Also, forward Serge Ibaka returns after averaging 13.2 points last season.
Presti said losing Westbrook in the early going was worth it for the peace of mind.
"Of course, both from the standpoint we'd like to have him on the floor as soon as possible, but in this case, although we lose a little bit of time, we gained a tremendous amount of confidence in the actual progression and recovery process of the knee itself," he said.
Information from The Associated Press was used in this report.