"Never good to walk out with any of your players walking off the field with a trainer," Astros manager A.J. Hinch said. "That was a definite bummer."
After forcing Josh Donaldson into a 4-3 groundout in the top of the fifth, McCullers started hobbling a bit and was promptly pulled from the game after being looked at by trainers.
"I just felt a little bit of discomfort," McCullers said. "That's why the trainers came out, and we just decided to err on the side of caution. Just get out of there."
McCullers went 4 2/3 innings, allowing home runs to Jose Bautista and Edwin Encarnacion to give Toronto a 2-0 lead. It was the first time this season that he allowed multiple home runs.
After starting the season on the disabled list with a sore right shoulder, McCullers has emerged as one of the Astros' most dominant pitchers. He leads the rotation in strikeouts per nine innings (11.8) and was coming off two straight games with 10 strikeouts.
McCullers said the injury is a little frustrating considering he was pitching so well up to that point.
"I didn't start the season how I wanted to, being on the DL and whatnot," he said. "I just felt like I was starting to hit my stride a little bit. We'll see how things go."
Jordan Ray is a reporter for MLB.com based in Houston. This story was not subject to the approval of Major League Baseball or its clubs.