McAllister was hurt on a screen pass in the third quarter of the Saints' 52-3 loss to Green Bay. An MRI exam Monday disclosed the seriousness of the damage.
McAllister had just 31 yards on 11 carries and caught three passes for 33 yards before leaving the injury.
McAllister has run for 335 yards with three touchdowns this season and was the team's second-leading receiver with 17 catches for 117 yards.
"We have confidence in the guys that we have behind him," said Saints Coach Jim Haslett. "That's why we signed Antowain (Smith) and (Aaron) Stecker."