Erat, acquired from the Nashville Predators along with Michael Latta for top prospect Filip Forsberg on Wednesday, was injured when he was hit into the boards by Florida defenseman Erik Gudbranson. After staying on the ice for a few minutes, Erat needed help to get to the dressing room and couldn't put weight on his right leg.
Gudbranson was assessed a five-minute penalty and a game misconduct on the play.
Erat was not on the Capitals' bench at the start of the second period. His return is questionable, according to a team spokesman.