Backup Ryan Finley came in at quarterback for Cincinnati.
Burrow has not had a recorded leg injury in his career, per Sports Injury Predictor.
Burrow threw for 203 yards and one touchdown on 22-of-34 passing before exiting. Instead, the future of Burrow's rookie season is now in question after the injury. Burrow was initially hit by Washington defensive end Montez Sweat, before Washington defensive end Chase Young fell onto his left leg on the play.
The Bengals held a 9-7 lead over Washington when Burrow was injured. Mixon (foot) was already ruled out for this Sunday's game at Washington.
Cincinnati had hoped Burrow was the franchise's next cornerstone quarterback when the Bengals selected the southeast OH product with the top overall pick in April's draft.
Burrow did not reveal the exact nature of his injury but said "see ya next year" in a tweet later on Sunday.
The No. 1 pick in the 2020 NFL Draft after winning the Heisman Trophy at LSU, Burrow has started each game of his rookie season for the Bengals.
If Burrow is out for an extended period of time, it will delay Cincinnati's rebuilding project that is centered on the rookie quarterback.
Mixon, a fourth-year player, suffered a right foot injury in the Week 6 loss at Indianapolis and has been week-to-week.