Ryan Donato, a 22-year-old forward, will be in the lineup regardless of the status of Marchand or DeBrusk. Donato hasn't played since Game 2 against the Toronto Maple Leafs in the first round April 14.
Tampa Bay leads the best-of-7 series 2-1.
RELATED: [Complete Lightning vs. Bruins series coverage]
Marchand and DeBrusk took maintenance days Thursday and did not practice.
"I believe both [Marchand] and Jake will play, but if not, there's a domino effect," Cassidy said. "... I just can't tell you what line [Donato] will play on."
Game 2 against the Maple Leafs was Donato's debut in the Stanley Cup Playoffs; he did not have any points and was on the ice for a goal against.
"Whatever they really need me to do," Donato said. "I've played a couple roles this year. I was in the back of the lineup where I was not getting that many minutes but I was trying to get shots on net, just create some offensive plays. Then if I have a lot of opportunities, I hope to create some goals, maybe get some assists as well."
Donato joined the Bruins on March 19 after finishing his season with Harvard University. He played 12 regular-season games and had three points (one goal, two assists) in his first game, against the Columbus Blue Jackets, and finished the season with nine points (five goals, four assists).
"When he was in the lineup, he was on pucks," Cassidy said. "He shot the puck. He obviously created offense for us. … Just some good steady play. Some energy. That's a bit of it. I think the last two games we needed a little more of that. We need some more pop in our lineup, a little more energy, enthusiasm and puck pursuit. He can add that."
Cassidy said he expects to use Donato on the second power-play unit.
