The 39-year-old Thomas was told he would lose playing time as a result of the slow start. On Saturday, he alluded to the decision having something to do with the 2009 option on his contract -- a claim the Blue Jays say was a non-issue. If Thomas had made 376 plate appearances this season, the $10 million option automatically would have kicked in.
"Obviously, reduced playing time is not something that he was interested in," Toronto general manager J.P. Ricciardi told The Associated Press. "In order to let him go forward and get on with his career, I think it's fair to do it at this point."
Thomas hit .277 with 26 homers and 95 RBIs last season for the Jays. The slugger also eclipsed the 500-homer mark in his tenure with the Blue Jays, and currently he is 18th on the all-time list with 516.
Thomas won a World Series title with the White Sox in 2005, but he was not on the postseason roster due to an injury.
"It's not the nicest way to do it," Guillen said of being released. "I think [he has] a nice career, a very nice career, and he's going to be in the Hall of Fame. And he's one guy you put a name at the top of baseball, and it's Frank Thomas."
Thomas has several White Sox records to his name, including all-time leader in runs scored (1,327), home runs (448), RBIs (1,465), extra-base hits (906), walks (1,466), total bases (3,949), slugging percentage (.568) and on-base percentage (.427).
It is unclear whether Thomas will look to continue the season with another team or explore other options, such as coaching.
"This is just part of the game," Guillen said. "This game has gotten so crazy, you never know what's going to happen."