Late in the second period, Phaneuf took the brunt of a hit from Buffalo forward Marcus Foligno on Nazem Kadri.
Kadri fell into Phaneuf's right leg, and the Leafs' captain went immediately to the bench putting no weight on his right leg.
Phaneuf was in distress on the bench and went down the tunnel for medical attention.
Foligno was given a kneeing penalty on the play, and Tyler Bozak scored on the ensuing power play.
Phaneuf was one of the final Leafs players out after intermission, but took the first shift of the third period.