Earlier in the day, the Giants listed Stewart as out of Sunday's game against the New Orleans Saints because of the injury.
The 5-foot-10, 240-pound Stewart signed a two-year, $6.8 million contract with the Giants in March after 10 seasons with the Carolina Panthers, where he was the team's career rushing leader with 7,318 yards and 1,699 carries.
Gallman has played well in the backup role with 11 total touches for 46 yards from scrimmage.
Stewart, who was the Panthers' No. 1 pick (No. 13 overall) out of Oregon in 2008, will be eligible to come off injured reserve in eight weeks.
The Giants also ruled wide receiver Cody Latimer out of Sunday's game because of a knee injury, and promoted wide receiver Jawill Davis, an undrafted rookie out of Bethune-Cookman, from the practice squad.
Davis, originally signed by the Giants on May 10, made 106 receptions for 1,692 yards and 10 touchdowns in his college career.