Head coach Jay Gruden predicted the injury will keep him out three to four weeks.
The 22-year-old had started all five games this year, tallying 10 tackles and one sack.
Allen emerged as one of the most talented prospects in the 2017 NFL draft after racking up 22.5 sacks across his final two seasons at Alabama. The versatile lineman has the potential to become a stalwart of the Redskins' front seven.
After originally being projected to land in the top five, he fell to Washington with the 17th overall pick due to injury concerns. More specifically, some teams were worried about his long-term outlook because of his surgically repaired shoulders.
The coaching staff could opt to use a rotation of players in an attempt to fill the void. Matthew Ioannidis, Stacy McGee and Terrell McClain all saw significant snaps in Week 6 and could see an uptick in playing time based on down, distance and package for the time being.
Ultimately, Allen possesses all the necessary tools to help bring stability to the Redskins defensive line in the years ahead. Injuries are a factor that could knock him off that course, but it's too soon to sound any serious alarm bells about his future outlook.