The team confirmed Chara was out just minutes before the opening faceoff.
The Bruins lost Game 1 on Thursday night 2-0. They have never come back in a series after losing the first two games.
The 6-foot-9 captain was the top-scoring defenseman on the Bruins with 14 goals and 30 assists. He did not miss a game all season before sitting out the finale to get extra rest for the playoffs.
The team said on Saturday morning that Chara was hospitalized during the night for dehydration, reportedly after flulike symptoms.
"Until our medical staff tells me he can't go, he's in tonight," Bruins coach Claude Julien said after the team's optional morning skate on Saturday.
Chara played 25 minutes in Game 1, then practiced Friday afternoon before being struck by the illness. He has missed just a total of 10 regular-season games, eight because of injury, in his five seasons with the Bruins.
"I think if it's up to him, he'll definitely play," Julien said in the morning. "The only people that'll hold him back are our doctors. That's the type of players you want on your team and he's got that attitude."
Some of Chara's teammates were not aware that the defenseman was hospitalized.
"I didn't know until last night," said forward Milan Lucic. "He's a battler. And I'm sure he'll do whatever he can to be able to get himself ready to play tonight."
The series shifts to Montreal next week for Game 3 and 4.