Thornton was removed from Monday's game against the Tigers after being struck on the left foot by a comebacker, Mike McGraw of the Chicago Daily Herald reports.
Thornton was deployed in the top of the 10th inning, but the first batter of the frame drilled the right-hander with a comebacker, forcing Thornton to exit the matchup. The 32-year-old was able to put some weight on his foot as he walked off the field and will likely be sent for further evaluation.