Thomas was in uniform and scheduled to bat fourth against the Minnesota Twins before the team made the announcement less than two hours before the start of the game.
The 40-year-old slugger had complained about soreness in his leg in recent days and acknowledged he had trouble pushing off with his right foot. He was mired in a career-long 0-for-23 slump before going 8-for-21 (.381) over his last six games.
Thomas left without speaking to reporters but Oakland manager Bob Geren acknowledged the injury could end Thomas' season. He missed 55 games while on the DL with a similar injury earlier this season.
"He is going to get further tests soon but that is a possibility," Geren said. "Being that's a similar feel and in the same spot, it's a possibility, yes."
Thomas is tied with Willie McCovey and Ted Williams for 17th place with 521 career home runs.
He began the season with Toronto but batted only .167 with three home runs and 11 RBI for the Blue Jays. Thomas signed with Oakland on April 24, four days after being released by Toronto. In 55 games with Oakland, Thomas hit .263 with five home runs and 19 RBI.
The A's also reinstated right-handed reliever Keith Foulke from the disabled list, called up outfielder Aaron Cunningham from Triple-A Sacramento, optioned left-hander Dan Meyer to Double-A Midland and transferred right-hander Andrew Brown to the 60-day DL.