Wells clutched at his knee and lower hamstring after going into third base standing after stealing the base in the sixth inning of Wednesday's game against Baltimore. Trainer George Poulis and manager Cito Gaston came out to check on Wells, who stayed in the game and scored when Scott Rolen singled to left. Wells did not come out on defense the next inning.
"Going to third he hyperextended his knee," assistant general manager Bart Given said Thursday. "When you hyperextend it, you put some strain on those tendons. It's not completely ruptured, which is good news, but he's obviously not able to play."
Toronto recalled outfielder Kevin Mench from Triple-A Syracuse to take Wells' spot on the roster. Mench will join the Blue Jays on Friday, in time for the start of a three-game series against the New York Yankees.
The 29-year-old Wells is batting .287 with nine home runs and 42 RBIs this season.
Wells, who is in the first season of a seven-year, $126 million contract extension, missed 26 games this season after he fractured his right wrist making a diving catch at Cleveland on May 9.
The news was better for right-hander Dustin McGowan, who left his start Tuesday after four innings and 66 pitches, saying the pain he had been feeling in his shoulder in his past few outings had become too much to bear.
McGowan has been pitching for some time with a tear in his rotator cuff, but an MRI taken Wednesday revealed no further damage since an examination late last season.
"It's very good news there's no new damage to the shoulder and we'll get started with the rehab next week," Given said. "Surgery doesn't seem like an option at this point."
McGowan, who is 6-7 with a 4.37 ERA in 19 starts, said he won't be completely reassured until Dr. Timothy Kremchek examines the latest MRI on Monday and compares it to last year's.
"You want to be happy but you never know for sure," McGowan said. "We'll see."
Given said McGowan, who worked a career-high 169 2/3 innings last season, will visit Kremchek once the swelling in his shoulder subsides to develop a schedule for his rehabilitation.