The injury is a setback in an otherwise encouraging rookie season for the third-round draft pick.
Gibson entered Monday having run for 645 yards and 11 touchdowns while catching 32 passes for 233 yards. He shredded the Dallas Cowboys on Thanksgiving Day, going for 115 yards and three scores on the ground in a 41-16 win.
Losing Gibson is obviously bad news for a Washington offense that's averaging 325.3 yards per game.
Alex Smith's comeback from a gruesome leg injury makes for an incredible story, but he has been somewhat limited upon his return to the field.
His 5.8 adjusted yards per attempt were tied for 28th with Daniel Jones, per Pro Football Reference. Per NFL's Next Gen Stats, his 4.7 average intended air yards are also the lowest in the league and more than a full yard below the next closest quarterback (Drew Brees, 5.8).
Washington has a below-average offense with everybody healthy. Take Gibson out of the mix and the team will likely struggle to move the ball against Pittsburgh.