Horton appeared to take a stick or puck up high while Boston was on the power play in the first overtime period. In pain, he immediately skated off the ice and went down the tunnel.
Horton had skated 32 shifts for 20:55 until he left the game. He assisted on Milan Lucic's goal that opened the scoring in the first period.
Bruins head coach Claude Julien didn't have a status update on Horton after the game, saying only that was still being evaluated.
Julien rotated through options alongside Lucic and David Krejci, Tyler Seguin replaced Horton on the Bruins' top line to start the second overtime. Horton was not on the bench.
Boston already came into the series without Gregory Campbell, who suffered a broken fibula in Game 3 of the Eastern Conference final. Kaspars Daugavins stepped into the lineup in Campbell's place.
Horton has seven goals and 11 assists in the playoffs for the Bruins.