The Rockets announced Westbrook will sit out Wednesday and Friday's games after an MRI revealed a strained quad.
According to Jonathan Feigen of the Houston Chronicle, Westbrook's status for the start of the postseason next week is uncertain.
Westbrook played 28 minutes in Tuesday's 123-105 loss to the San Antonio Spurs after sitting out the previous two games. He scored 20 points on 8-of-17 shooting with six assists and five rebounds.
Any injury to the 31-year-old Westbrook is a major blow to Houston's title chances. He has been excellent in his first season with the Rockets, averaging 27.2 points, 7.0 assists and 7.9 rebounds per game.
Any concerns about their ability to coexist have been allayed. Harden hasn't missed a beat and is an MVP finalist with averages of 34.3 points, 7.5 assists and 6.5 rebounds this season.
Both also offer the Rockets flexibility in how they attack defenses. Harden is incredible in the half court, beating his defender in isolation or running the pick-and-roll to perfection. Westbrook is a blur in transition, and his ability to push the pace even off an opponent's made basket allows the Rockets to get some easy buckets early in the shot clock.
Westbrook also remains adept at getting to the rim and finishing, giving the Rockets a deep well of players who can both create their own shot and facilitate for the rest of the team.
With Westbrook out, look for Eric Gordon and Austin Rivers to play even bigger roles for the Rockets. Despite that solid guard depth, however, Houston will be hard-pressed to advance in the postseason without Westbrook on the floor.