Ian Rapoport of NFL Network reported Smith will undergo surgery after breaking his tibia and fibula.
Albert Breer of The MMQB noted Smith's right leg was placed in an air cast right after suffering the injury.
Former Redskins quarterback Joe Theismann, who famously suffered a serious leg injury on the same day in 1985 (h/t Darren Rovell of ESPN.com), expressed his condolences:
Smith hasn't appeared in fewer than 15 games in a season since 2012. Entering Sunday, he had completed 64.1 percent of his passes for 2,045 yards and 10 touchdowns with three interceptions this season. Smith was 12-of-27 for 135 yards with two interceptions Sunday.
With Smith out, the onus will be on Colt McCoy to play his first meaningful snaps since 2014—when he played five games, including four starts, and threw for over 1,000 yards.
Since then, McCoy has attempted 11 passes, all of which came in Week 17 of the 2015 season. On Sunday, McCoy completed six of his 12 attempts for 54 yards and a touchdown in a 23-21 loss.
Washington will need him to shake off the rust in short order if it wants to remain perched atop the NFC East.