That means Westbrook won't make his scheduled start on Thursday. And there's no guarantee that he will return to the mound anytime soon. With only 23 games remaining and the Cardinals in a dogfight for a playoff spot, the timing couldn't be worse.
Westbrook admitted, too, that "it's a possibility" his 2012 season could be over.
"It's pretty annoying," Westbrook said. "It's pretty sore today."
Westbrook felt an uncomfortable pull in his right side as he delivered a 1-0 pitch to Brewers third baseman Aramis Ramirez in the fifth inning on Saturday. Westbrook said he considered calling out a trainer at the time, but instead opted to pitch his way through the discomfort.
He threw eight more pitches to finish the inning. Westbrook was pinch-hit for in the bottom half of the frame.
"Something didn't look right early on, but he didn't feel anything yet," manager Mike Matheny said. "He was just fighting to really find his [arm] slot. It was one of those starts that you just fight your way through. He was most likely coming out [for a pinch-hitter] anyway. But obviously that made that decision very easy."
Westbrook is slated to undergo a series of tests on Sunday and/or Monday to determine the severity of the oblique injury. Healing times for this type of injury can vary drastically. Westbrook has been shut down from an oblique strain twice before, though both of those were on his left side, which does endure more stress during the right-hander's pitching motion.
Westbrook missed approximately six weeks recovering from the side injuries in 2007 and 2008.
Though the oblique injury can't be blamed, Westbrook has not pitched well as of late. A string of 13 straight starts of at least six innings ended on Aug. 23, and Westbrook hasn't stuck through the sixth in four starts since. Opponents have hit .400 off Westbrook in those four outings, and he has given up 17 runs in 20 1/3 innings.
On the season, Westbrook is 13-11 with a 3.97 ERA. Though he dealt with a number of injuries during his time in Cleveland, Westbrook has not missed a start since joining the Cardinals in July 2010.