It was not clear whether he would play in Game 2 against the Chicago Blackhawks on Saturday night.
"If he's feeling better tomorrow, he's in the lineup," Julien said. "Simple as that."
Horton did some rushes with usual linemates Milan Lucic and David Krejci, but not all of them. He was not made available to reporters.
Julien and Bruins players called it an encouraging sign that Horton was able to skate and take part in Friday's practice.
If Horton is able to play, Boston's lines could go back to normal. It wasn't clear who would step into the lineup if he cannot.