Horton has been sidelined for 36 games after suffering the second known concussion of his career in January.
"If he were to be cleared at some point, he would need at least a couple of weeks to get back," Chiarelli said on 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 collected 17 goals, 15 assists and 54 penalty minutes in 46 contests this season.