The Eagles quarterback has left for the locker room, escorted by team medical personnel.
Replays showed Foles grimacing after the hit and again on the sideline as he was in obvious discomfort.
The Eagles list him as questionable to return with a chest injury.
With a 17-0 lead and Washington doing nothing on offense, it is unlikely Foles re-enters even if he is cleared to go back in.
In the fourth quarter last week, Foles left for one play after an illegal hit by Texans linebacker Jadeveon Clowney.
Nate Sudfeld has replaced Foles, who tied an NFL record with 25 consecutive completions earlier in the game. Foles is 28-of-33 for 221 yards with two touchdowns and one interception.