Thornton's contract was purchased by Toronto on Tuesday.
With the Rule 5 draft looming, Thornton was added to the 40-man roster to ensure protection. The 25-year-old prospect made 24 starts at Triple-A Fresno last year, posting a 4.42 ERA and 1.20 WHIP with a 122:31 K:BB over 124 frames. Thornton figures to have an opportunity to battle for a spot in the starting rotation during spring training given the Blue Jays' lack of pitching depth.