The 24-year-old Kiwi centre suffered a second straight injury that forced him to leave the game prematurely during the Thunder's hard-fought 106-101 home victory over the Sacramento Kings that improved them to 40-29 for the season.
Westbrook had made 5 of 31 since All-Star and hit 2 of 3 attempts in a 17-point, 10-rebound, 11-assist triple-double.
Thunder center Steven Adams left the game in the third quarter with a left hip contusion and did not return.
Paul George and Carmelo Anthony led the Thunder with 21 points each, with George hitting four three's and Anthony five, including the first three baskets of the game for OKC.
Following the Atlanta match up, the Thunder take on the LA Clippers, who are battling for a playoff spot in the West, and the top two teams in the East in the Toronto Raptors and Boston Celtics.
The Kings were led by 19 points from Bogdan Bogdanovic and 15 apiece by Justin Jackson and Garrett Temple. George had 14 points and Anthony 13 for the Thunder.
The Thunder's defense snoozed for long stretches until the fourth quarter. He got his 10th assist on a pass to Jerami Grant for a layup and his 10th rebound with just over 3 minutes to play.
Sacramento cut its deficit to 104-101 on a pair of free throws by Bogdanovic with 23 seconds left, but Brewer made two free throws with 17 seconds left to put it out of reach.
Thunder: G Terrance Ferguson returned after missing Saturday's game against San Antonio while in concussion protocol.
But the 3-point shot - a fickle weapon this season for Oklahoma City - was dependable on a night when the Thunder badly needed it.