Foster has been trying to play through the injury since their game against the Rams, but it’s too painful for him to continue.
The Redskins will turn to Will Compton and Martrell Spaight in Foster’s absence, but it would not be surprising to see them bring in another linebacker.
Foster, 28, is a former third-round pick of the Buccaneers back in 2011. He played out his rookie contract in Tampa Bay before signing on with the Redskins 2015.
Foster is currently in the final year of his two-year, $2.5 million contract and stands to make a base salary of $1.22 million this season before becoming an unrestricted free agent next year.
In 2017, Foster has appeared in five games and recorded 31 tackles, a half sack, an interception, a fumble recovery and a pass defense for the Redskins. Pro Football Focus has him rated as the No. 44 linebacker out of 79 qualifying players.