Allen, drafted with the second-round pick Philadelphia acquired from Washington in the Donovan McNabb trade, is the fourth defensive starter to go down for the season.
A potential fifth, middle linebacker Stewart Bradley, is out with a dislocated elbow and may not return until the playoffs.
He'll join rookie defensive end Brandon Graham, defensive tackle Brodrick Bunkley and cornerback Ellis Hobbs on injured reserve. Rookie Kurt Coleman, a seventh-round pick, takes Allen's starting spot.
Copyright 2010 by The Associated Press