The Cubs placed Thornton on the 15-day injured list Saturday with left ankle soreness, Patrick Mooney of The Athletic reports.
Thornton has been one of the more reliable members of a shaky Cubs bullpen this season, but his move to the IL will prevent him from pitching in a game until the final day of the regular season. Edward Cabrera (finger) was reinstated from the injured list in a corresponding move and will join Chicago's bullpen to fill the void left by Thornton.