Bruins general manager Peter Chiarelli recently called Horton's possible return a "long shot" after the winger missed the final 36 games of the regular season after suffering his second concussion in a seven-month span in January.
He recently returned to the ice and had been skating on his own, attempting to return for the postseason.
Horton suffered the injury in the second period against Philadelphia on Jan. 22 when he was on the receiving end of a late hit by the Flyers' Tom Sestito. Horton stayed in the game for the remainder of the period but sat out the third. That was the last time he played.
Horton suffered a severe concussion in Game 3 of the Stanley Cup finals last spring and missed the rest of the series. He also struggled at the beginning of this season and admitted that effects of the concussion may have been partly to blame.