It's the same hamstring that kept Wells out for a month last season. The two-time All-Star was scheduled for an MRI late in the day and the team was waiting for results.
"Right now, preliminarily, I'd say he's going to miss four weeks," Blue Jays general manager J.P. Ricciardi was quoted as saying by MLB.com.
Wells missed 52 games due to injuries in 2008. He broke his left wrist making a sliding catch early in the season, then returned to the disabled list July 10 with a strained left hamstring.
A three-time Gold Glove winner, Wells batted .300 with 20 home runs and 78 RBIs in 108 games last year. He spent the offseason working with a personal trainer, hoping to strengthen hamstrings that have bothered him for years.
Toronto manager Cito Gaston said he thought Wells was hurt running the bases.
"I was on another field," Gaston said, "but I noticed he wasn't doing the cutoffs and relays so it must have happened during that time."
Gaston said the only positive thing about Wells' injury is that it happened early in spring training.
"He's got a chance to be ready to start the season," possibly as a designated hitter, Gaston said.
It's the latest setback for Wells, who signed a $126 million, seven-year contract before the 2007 season. Last weekend he was told insurance costs would keep him off the United States squad in the World Baseball Classic. Team USA plays its WBC opener against Canada on March 7 in Toronto.
Gaston said Jose Bautista could replace Wells in center field Wednesday when the Blue Jays play the New York Yankees in their spring training opener. Bautista played center in the Caribbean Series this month and has played 133 games in the outfield, 65 in center, during his five major league seasons.
LHP Mike Maroth, who hasn't pitched in the majors since 2006, said he felt pain in his left knee after last Tuesday's workout and acknowledged he might need surgery. "The cartilage is not going to heal on its own. It's just a matter of what I'm able to do," he said. Maroth has thrown briefly the past two days. ... After Tuesday's workout, the Blue Jays break camp and move to Dunedin Stadium for the rest of spring training. ... Brett Cecil is scheduled to start Wednesday against the Yankees, with fellow rookie Scott Richmond starting Thursday against Philadelphia, Matt Clement on Friday against Detroit and Roy Halladay on Saturday against Atlanta.