Foster was injured on the third snap of the Redskins' first organized team activity practice session of this off-season. He was carted off the field and later underwent an MRI exam. He will seek a specialist to determine if his knee suffered more damage beyond the ACL tear, according to ESPN.
Washington head coach Jay Gruden told reporters that Foster stepped on offensive lineman Tyler Catalina's foot as he rushed a gap during a drill. He laid on the ground, smacking the turf as members of the Redskins' medical staff attended to his leg. He was brought to tears after the significant injury.
"He wouldn't be laying down unless something was wrong," Gruden said after practice. "We'll hope for the best right now until we get the results, but, very concerning for sure. I'll just have to wait and see before I get too emotional about it.
"I don't know how to process it. We've had some bad luck over here for the last couple years, but this one here takes the cake because this was a non-contact drill and there was no contact involved in it. He just landed funny."
The San Francisco 49ers drafted Foster in the first round of the 2017 NFL Draft. The franchise waived him in November before he was claimed by the Redskins. The 25-year-old linebacker was suspended two games last season for violating the NFL's policy on personal conduct and substance abuse.
Foster was fined in April after a November arrest relating to allegations of domestic violence. Florida prosecutors dismissed a misdemeanor domestic violence battery charge against Foster in January.
Washington continues OTAs on Tuesday before returning Thursday for voluntary workouts. The Redskins begin mandatory minicamp June 4.