Horton was injured with about seven minutes left in the first overtime of Game 1 Wednesday after a seemingly routine push to the chest of Blackhawks defenseman Niklas Hjalmarsson.
Bruins coach Claude Julien addressed Horton's condition Thursday. The Blackhawks won Game 1, 4-3, in triple overtime.
"You look at what he's accomplished and he's a big part of our team," Julien said. "He had a bit assist on the [Milan] Lucic goal on Wednesday. He's listed day-to-day right now, so we'll see."
Immediately after shoving 6-foot-3, 207-pound Hjalmarsson, Horton (6-2, 229) grimaced and bent over in obvious pain before skating to the bench. He quickly retreated to the dressing room and did not play again, leaving the Bruins one forward short in what would turn out to be the fifth-longest game in the history of the Cup Final (112:08).
Forward Tyler Seguin took over most of the shifts on the top line in Horton's absence, but changes are likely in order for Game 2 if Horton, a right wing, is unable to return.
"I thought [Seguin] skated, had some chances and has the speed and skill to be up there with [David Krejci and Milan Lucic]," Julien said. "For him, it's an opportunity right now, so he did a great job and we'll see where everyone stands on Saturday."
Bruins defenseman Torey Krug said he thought Seguin stepped up when the team needed him following Horton's injury.
"[Horton's] a guy who has been coming up big in the playoffs so far, so to not have him for the rest of that game definitely hurt us," Krug said. "But we have confidence in our guys. [Seguin] stepped up and played a great game in the second half."
Horton assisted on Boston's first goal, by Lucic in the first period, but was stripped of the puck prior to Chicago's Brandon Saad scoring his first goal of the playoffs in the second.
Horton, who had one assist in 22:28 of ice time, missed the last five regular-season games with an upper-body injury sustained in a first-period fight with Pittsburgh Penguins forward Jarome Iginla on April 20.
Horton has seven goals, 18 points and a League-high plus-22 rating in 17 playoff games.
"It [stinks]," Lucic said. "Everyone knows how big of a player he is for us, especially in the playoffs. He's also shown that he can score big goals at big times."
Horton has 15 goals, 35 points and a plus-33 rating in 38 playoff games.
"If someone is out, then someone else is in, and we would expect them to do the job," Bruins defenseman Andrew Ference said. "At this time of the year, depth is what gets you this far. You continue to succeed with depth, and the responsibility falls on those guys to assume a role if they have to, but that's the same for any other team."
The Bruins were without Horton during last year's first-round series loss to the Washington Capitals. He missed all seven games, along with the final 36 games of the regular season in 2011-12, with a concussion. He was taken off the ice on a stretcher during the first period of Game 3 of the 2011 Stanley Cup Final against the Vancouver Canucks after a collision with defenseman Aaron Rome. Horton missed the remainder of the series, which the Bruins won in seven games.
When healthy, Horton is a proven commodity alongside wing Lucic and center Krejci. During Boston's four-game sweep of the Pittsburgh Penguins in the Eastern Conference Final, Horton had two goals and five points.