The club has listed the 30-year-old native of the Dominican Republic as day-to-day, one week ahead of Toronto's season opener against the Tampa Bay Rays (4:10 p.m. ET). Both the team and Reyes had been insisting the injury wasn't severe.
Reyes, a six-foot, 195-pound switch-hitting speedster, has dealt with hamstring issues in the past and hurt himself during Saturday's game against the Detroit Tigers.
The four-time All-Star played in just 93 games last year in his first season in Toronto with a severe ankle sprain. He joined the team in blockbuster trade with the Florida Marlins in the off-season ahead of the 2013 campaign.
The former National League batting champion hit .296 with 10 home runs, 37 RBI and 15 steals last year.