Horton 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 has been skating on his own, but it's unlikely he'll return for the postseason.
"If he were to be cleared at some point, he would need at least a couple of weeks to get back," Chiarelli said during a 20-minute conference call Sunday. "He's quite far off right now. I know he's skating, but he's having little bouts here and there of post-concussion symptoms. It's a long shot. I'm not going to rule him out, yet, but it's a long shot."
Horton's not the only health concern as the Bruins head into the playoffs.
Defensemen Johnny Boychuk (knee) and Adam McQuaid (eye) are listed as day to day, but both should be ready for Boston's first-round series against the Washington Capitals, which begins Thursday at TD Garden.
Backup goalie Tuukka Rask, who missed the final 19 games of the regular season because of a groin/abdominal injury, has been on the ice but he remains questionable for the Capitals series.
"I think Tuukka is a little more than day to day," Chiarelli said. "He's been pretty aggressive with his rehab. We'll see how he goes the next couple of days, but we certainly have a capable backup if we need him in Anton Khudobin."
After Rask was injured on March 3, Khudobin got hurt while playing for the Providence Bruins, forcing Chiarelli to sign veteran Marty Turco to serve as Tim Thomas' backup. However, Turco is not eligible to play in the postseason because he was signed after the NHL's trade deadline.
Chiarelli said Turco would remain in Boston for the next few days, but will return home near the end of the week.