Losing the heroic Gregory Campbell hurt the Bruins, but it was a setback they were able to absorb fairly well. The loss of Nathan Horton would be an entirely different matter. It was a devastating night for the Bruins Wednesday in the magnificent opener of the Stanley Cup finals, and not just because of the way it ended one minute before midnight local time — when a shot pinballed off two Blackhawks and past Tuukka Rask with 7:52 left in the third overtime, giving the hosts a 4-3 win. The insult for the Bruins was knowing they’d missed so many glorious opportunities to win this game and seize control of the series. And the left shoulder injury to Horton, if serious, will make it far more difficult for the Bruins to bounce back. While coach Claude Julien said only that the big winger is day-to-day, it seems possible he could play in Game 2 tomorrow night. “We’re keeping our fingers crossed,” said Julien, meeting with reporters at the B’s swanky lakefront hotel. Horton’s injury first occurred April 20 when he fought Pittsburgh’s Jarome Iginla at the Garden. Iginla hung on Horton’s left arm and yanked him awkwardly down to the ice. Horton missed the final six regular season games. He returned for the start of the playoffs and has played tremendously well alongside linemates David Krejci and Milan Lucic. Horton has 7-11-18 totals, three game-winning goals and a league-leading plus-22. But midway through the first overtime Wednesday, he had a seemingly minor bump from Chicago defenseman Niklas Hjalmarsson, left the game and did not return.