According to ESPN's Lisa Salters (h/t Ian Rapoport of NFL Network), X-rays revealed a fractured right fibula for McCoy, ending his season.
Mark Sanchez, who signed with Washington on Nov. 19 following Alex Smith's season-ending leg injury, replaced the injured McCoy under center.
The team only had two quarterbacks active for the game, so tight end Jordan Reed served as the emergency quarterback, according to NBC Sports' JP Finlay.
McCoy left the game having gone 4-of-4 for 50 yards, zero touchdowns and zero interceptions and with Washington trailing 7-3. On Sanchez's first play of the game, though, former NFL MVP Adrian Peterson broke off a 90-yard touchdown run:
Washington was leading the NFC East at 6-3 when Smith went down. McCoy was unable to steer his team to a victory over the Houston Texans in relief of Smith in Week 11 and also dropped his first start of the season a week ago against the Dallas Cowboys. As a result, Washington (6-6 after a 28-13 loss Monday) has lost its position in the wild-card hunt.
While the 32-year-old Sanchez has a pair of AFC Championship Game appearances on his resume, Monday marked his first regular-season action since the 2016 season, when he was a member of the Cowboys. The former fifth overall pick has not been a full-time starter since 2012, his final year with the New York Jets.
He has played in just 15 games since the end of the 2012 season.
Now that the team has to turn to a quarterback it signed just two weeks ago, Washington will likely try to lean on Peterson even more. Sanchez's Jets career was plagued by turnovers, so Washington will have to hope the veteran can protect the football and let the ground game carry the workload.