Defenseman Matt Grzelcyk, who has been out since Game 2 with a concussion, is also a game-time decision.
The Bruins have been mum on details about Chara's health -- only calling it a "facial injury" -- after he was hit with a deflected puck in the second period of Game 4 and bled profusely. Multiple reports suggest that Chara is dealing with a broken jaw.
Team doctors did not clear Chara to return for the third period of Game 4, though he sat on the bench because he wanted to be with teammates.
"The first thing you worry about is a concussion, because then you're in the protocol," Cassidy said. "We avoided that, Z avoided that, so that was good news. Then it's a matter of what's next? So I guess yesterday we had a better idea and then this morning he was here, obviously, he was on the ice. And now it's just real."
Cassidy said that first, Chara needs to get the OK from team doctors, then it will be up to the defenseman on whether he can play.
An NHL spokesperson said Chara is unable to speak clearly right now. He gave written answers to a few questions from reporters on Thursday.
Asked how he weighs the risk of future injury when deciding whether to play, he responded: "You don't think about that. You think about playing. You don't go into a game thinking you might get hurt."
"Much is made of his professionalism, his toughness, his approach," defenseman John Moore said. "But until you see it in the flesh, you don't really have an appreciation for it. The guy is 42. When I'm 42 I'm certainly not going to be the first guy in the gym weighing all my food, squatting the most on the team. You throw in what he's going through, that's something I'll tell my kids about. When they wake up with a fever and they don't want to go to school. Those are life lessons I'll carry long beyond hockey that I'm grateful to have with Z."
Said Brad Marchand of Chara: "He's willing to play with one leg, one arm. It doesn't matter. He's out there working as hard as he can and sacrificing his body because he knows, at the end of the day, you win."
The Bruins have considered several contingency plans if they are without Chara, Grzelcyk, or both. Cassidy said on Thursday that dressing seven defensemen and only 11 forwards was still an option his staff was considering.
Chara is a left-shot defenseman and the Bruins don't have many experienced understudies on that side.
Rookie defenseman Urho Vaakanainen, a left-shot defenseman, practiced with the main group on Wednesday, although Cassidy said it was a "long shot" for Vaakanainen to play. The 20-year-old Finn played in only two regular seasons games for the Bruins.
"That would be a big ask," Cassidy said, noting Vaakanainen, who played a majority of the season in the AHL, has not been around the NHL club much. "A real big ask."
Veteran Steven Kampfer is a more likely candidate to plug in on the blueline. Kampfer, a right-shot defenseman, played in two Stanley Cup playoff games this spring -- including subbing in for the suspended Charlie McAvoy in Game 1 of the Eastern Conference Final. Kampfer scored a goal in that game.
"Whoever comes in, it's kind of a seamless transition," Kampfer said on Wednesday. "I think that's been our identity of our team this year, next man up and making sure that you're ready to go."
The 42-year-old Chara has a goal and four assists with a plus-12 in 20 playoff games this spring. He typically plays on the top pairing with Charlie McAvoy, and is an important player on the penalty kill. Chara played over 24 minutes in Games 2 and 3.
Grzelcyk spoke with reporters on Thursday and said he was still in concussion protocol, but is hopeful to be cleared before the game.
"I'm not really sure how it works," Grzelcyk said. "I'm just taking it day-by-day."
Grzelcyk was knocked out of Game 2 after being boarded by Blues forward Oskar Sundqvist. Grzelcyk has not played since and only began skating with the Bruins players on Wednesday. Sundqvist received a one-game suspension for the hit.
Grzelcyk, who has averaged 19:22 in his 19 playoff games this spring, declined to comment on the play on Wednesday, saying the league handled it and he would leave it at that.