Quinton Mayo of NBC Sports Washington said Westbrook "suffered a major setback" in his quad and is expected to be out for at least three-to-four weeks.
Through seven games with the Wizards, the 32-year-old has averaged 19.3 points, 11.3 assists and 9.7 rebounds. However, that quad injury took him out of play Monday.
Even with Westbrook, who moved from the Houston Rockets ahead of this season, the Wizards were struggling early. They got off to a 3-7 start with the fourth-worst defensive rating in the league before it was announced the UCLA product would miss time.
The Wizards will look to Ish Smith, who has averaged 5.9 points and 4.0 assists through 11 games, while Westbrook is sidelined.