Left-hander Ryan Borucki, who will start Monday against the Cleveland Indians, was reinstated from the 60-day injured list in a corresponding move.
Thornton, 25, has made 21 starts for the Blue Jays this season and has a 3-7 record with a 5.45 earned-run average. His last start came on July 20 against the Detroit Tigers, where he gave up four runs over three innings in an eventual 7-5 win.
Borucki, also 25, has missed the entire season with a left elbow injury he suffered in spring training. Last season he made 17 starts with the Blue Jays, posting a 4-6 record with a 3.87 ERA.
“I’m just going to be treating it like any other start,” Borucki said Friday of his first start of the season. “But I’m going to be anxious. I’m going to be anxious to get back to Toronto and pitch again.”