It's hard to see on the video what exactly happened on the play, but Horton was in noticeable discomfort.
Boston coach Claude Julien had no update or diagnosis on Horton in his press conference after the Bruins 4-3 loss in triple overtime.
"Our doctors have not finished their diagnosis," Julien said.
Horton immediately went to the Bruins' dressing room for the remainder of the first overtime and was replaced by Chris Kelly on the Bruins' top line for the last shift of the period. Tyler Seguin came out with the top unit to start the second.
Horton is the Bruins' second-leading scorer in the postseason and played a pretty solid game up until the injury. If he's out for any period of time, it would be a very tough blow for Boston. The top line with Milan Lucic, David Krejci and Horton has been lights out in the postseason.