The Mariners made the roster moves Monday. Montero had to be reinstated from the suspended list with Seattle's season now complete, but placed Seattle over the limit on the 40-man roster. Hart, who will be a free agent in the offseason, was designated to clear the spot.
Montero was placed on the suspended list Sept. 1 after a heated confrontation with cross-checker Butch Baccala in the stands at a minor league game in Boise, Idaho. Montero was on a rehab assignment.
Hart hit .203 with six homers and 21 RBIs in 68 games after signing an incentive-laden one-year deal in the hopes of restarting his career after missing the 2013 season following knee surgery.