The powerful back suffered the injury while being tackled on a 34-yard run in Thursday's preseason opener versus the New England Patriots. He limped to the sidelines and was removed from the game.
It was reported early Friday that the initial diagnosis was a sprained MCL, but a subsequent MRI revealed the full extent of the injury.
Guice posted a positive message on Twitter after learning of the diagnosis.
A first-round talent, the Redskins selected Guice with their second-round pick in April's draft. He was projected to become the team's starting running back and help turn around a unit that finished 28th in rushing in 2017.