Game 2 is Saturday at the United Center.
"Keeping our fingers crossed," Julien said at the team hotel. "Hopefully he'll be back next game."
The Bruins did not practice Thursday after playing close to six periods of hockey in Game 1, ultimately falling 4-3 in triple overtime.
During a Bruins power play in the first overtime, Horton was battling for position with Blackhawks defenseman Niklas Hjalmarsson in front of the Chicago net when he appeared to hurt his shoulder. The play continued, but Horton skated to the bench hunched over in pain and went right to the locker room. He did not return.
Horton missed the last five games of the regular season after sustaining an upper-body injury in a fight with Penguins forward Jarome Iginla on April 20. Horton returned for Game 1 of the first-round series against the Maple Leafs and has played in every game since. According to a WEEI.com report citing a source close to Horton, he has been taking injections before playoff games to deal with a left shoulder injury sustained in the Iginla altercation and aggravated the lingering ailment in Wednesday's Game 1.
If Horton's injury is serious enough to keep him out of the lineup, it will be a major loss for the Bruins because the top line of Milan Lucic, David Krejci and Horton has been a powerhouse in the Stanley Cup playoffs, combining for 57 points. Horton has seven goals and 11 assists for 18 points in the postseason.
"He's been good," Julien said. "You look at his statistics and you look at what he's accomplished, even with Lucic's first goal, it was a great heads-up play by [Horton]. He's been really good."
If Horton is not available for Game 2 and beyond, Julien does have some options. When asked who he would want to see in the mix, the coach said he'd rather wait to see Horton's prognosis first.
"I'm not going there because that decision's not made," Julien said.
Tyler Seguin replaced Horton for the remainder of Game 1 and played well alongside Krejci and Lucic. Seguin was skating and creating chances. Although Julien wouldn't say whether he'd keep Seguin on that line, the coach has been pleased with the forward's play.
"He's been skating well," Julien said. "To me, right now the only thing he needs to do is to be able to finish. If he can finish, it'll certainly help his confidence, help our hockey club. I'm not criticizing his work ethic because he's competing hard and he's got some chances. Those things are certainly positive, and there's only one thing left to do, and you hope for his sake, and our sake, that it comes along."
Energy-line left winger Daniel Paille is versatile and can play the right side on that top line if needed. He brings speed and grit and could fit in well. Plus, he has experience playing on the top lines.
"We've worked everybody hard," Julien said. "They've been pushed and they've done extra -- forwards and Ds included. There's nobody in that extra squad that isn't ready to step in right now."