"We've made the determination to shut him down for the remainder of the playoffs," Bruins general manager Peter Chiarelli said. "We felt that it wasn't in the long-term interest of Nathan to be having this specter hanging over him of trying to come back during this playoff season. He made one step forward and two steps back. We just made the determination upon consultation with our doctors, with Nathan, that it would be prudent to shut him down. He'll continue to rehab for next year."
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.
Chiarelli was asked how Horton took the news.
"I think he was relieved," Chiarelli said. "He obviously thought it was best, and ultimately he would have to agree with it, and he did. With Nathan, he gets within the group and he looks back at his contributions this past year, and in the last playoffs, and he starts getting anxious. That probably compounds it, too. I think he felt a sense of relief.
"Talking to Nathan over the course of the last couple of weeks, he doesn't look bad. He looks good, actually, but he gets these bouts of fogginess and doesn't feel right and you have to be very careful."
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.
"We lose a pretty good player, but then we haven't had him for quite a long time now," Bruins coach Claude Julien said. "He's had some setbacks lately where you could tell that he was feeling the pressure of trying to come back for the playoffs and when you're not feeling well, that's not right."
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.
The Bruins and Chiarelli are both confident Horton will be able to continue his career next season.
"Yeah, I am, but who knows," the GM said. "Going into two weeks ago, if you had asked me about him playing in this playoffs, I would have said 'There's a chance.' Then he had a couple of setbacks and you have to take a step back and look at the whole thing and that's what we did."
"He would be improving and then he would have some symptoms," Chiarelli explained. "They weren't huge symptoms, but they would always come up at some point after three, four, five or six days of positive stuff. It was a frustrating exercise for Nathan. It was a frustrating exercise for us because we've been through this rehab before with players.
"I've seen all kinds of rehab patterns now. Usually you see when the player has color and is animated, you think he's turned the corner. Then they have a bout of post-concussion symptoms. They manifest themselves in different ways and with Nathan sometimes it would be a fogginess, sometimes he wouldn't feel right and sometimes there would be a big headache. But it was always after three, four or five days of positive progress."
The Bruins will have to compensate for losing what Horton brings to the ice.
"I think I have to be physical no matter what," Milan Lucic said. "But he definitely makes it easier, I'm not going to lie. He's a big body, he's got such great speed and we all know about his scoring touch."
Information from The Associated Press was used in this report.