Westbrook threw a 2-1 pitch to Blue Jays catcher Sal Fasano, who fouled it off. The right-hander then stood on the mound as trainer Lonnie Soloff, manager Eric Wedge and pitching coach Carl Willis rushed out to see what was wrong.
After a brief conference, Westbrook headed to the dugout. The team described Westbrook's injury as "left abdominal tightness."
He was replaced by Fernando Cabrera.
Westbrook entered the game 1-2 with a 7.96 ERA. Earlier this season, he signed a three-year, $33 million contract extension through 2010.
Westbrook's setback comes just as the Indians are about to get Cliff Lee back. The left-hander has been sidelined with a strained abdominal muscle since the start of training camp and is expected to be activated on Thursday and start against the Blue Jays.
The 29-year-old Westbrook has been remarkably durable the past three years, making at least 30 starts and pitching no fewer than 210 innings each season. He hasn't been on the DL since 2002, when he sat out the final month with elbow inflammation.
If Westbrook has to miss any time, the Indians could recall right-hander Fausto Carmona from Triple-A Buffalo. The right-hander was optioned to the Bisons on Tuesday. In his last start, he pitched a career-high 8 1/3 innings in a win over Baltimore.