Foster is believed to have incurred additional damage to his knee and is meeting with medical specialists to determine the full extent of the injury, sources told Ian Rapoport of NFL Network.
Foster was reinstated to the Redskins' roster in April after the NFL determined it had no grounds to suspend him following its review of a domestic violence complaint.
The third-year defender was acquired last season via waivers from the San Francisco 49ers, who cut ties with him amid troubling allegations.
Foster appeared in only six games last year, compiling 29 tackles for the 49ers, but he flashed excellence as a rookie, posting 72 tackles in 10 games.
The 25-year-old was a first-round pick of the 49ers in 2017, and he was expected to anchor the Redskins' linebacker corps in 2019.