Gallup, who received the results of his MRI on Monday, suffered the injury in the second quarter of Sunday's loss to the Arizona Cardinals and left AT&T Stadium on crutches. His left knee appeared to buckle as he turned to catch a 21-yard touchdown pass from Dak Prescott.
Gallup, who is scheduled to be an unrestricted free agent after the season, had 35 catches for 445 yards and 2 touchdowns as the No. 3 receiver opposite Amari Cooper and CeeDee Lamb. He had missed seven games after suffering a calf injury in the season opener.
ESPN's Todd Archer contributed to this report.