According to a source with direct knowledge of the situation, Nathan Horton has been dealing with chronic left shoulder subluxation, which caused him to leave Wednesday’s Game 1 of the Stanley Cup finals during the first overtime. The injury was originally suffered when Horton fought Penguins forward Jarome Iginla on April 20, with Horton missing the final five games of the regular season before returning for the playoffs. Horton has received shots prior to each game to deal with the pain, and though his shoulder has popped out of its socket at points during the playoffs, Wednesday’s occurrence, when he bumped into Blackhawks defenseman Niklas Hjalmarsson in front of the Chicago net on a Bruins power play, was the most painful thus far. His shoulder was popped back in after he left the ice, but he was in too much pain to return to the game. The injury will not require surgery until the offseason, when he is likely to receive a procedure to tighten the socket so the shoulder stops popping out. It is unknown whether Horton will play in Game 2 on Saturday, with the team labeling him “day-to-day,” though he would not have been able to play had the Bruins had a game on Thursday. Horton is second among all skaters with 18 points (seven goals, 11 assists) this postseason and will be an unrestricted free agent at season’s end. His agent offered no comment on the situation.