The Lightning took a 3-1 series lead.
Donato played on the third line with Sean Kuraly and Brian Gionta. Cassidy said he was hoping the rookie could provide a spark for the team after the team's lackluster effort in Game 3 in Boston on Wednesday night.
"We need some more pop in our lineup. We need some more energy, enthusiasm and puck-pursuit," Cassidy said. "I think he can add that."
Donato, a star at Harvard and on the U.S. Olympic hockey team, was a late-season call-up for the Bruins. He had five goals and four assists in 12 regular-season games.
In his only playoff appearance, Donato played 9 minutes, 24 seconds in Game 2 of the first-round series against Toronto. He had three shots on goal in a 7-3 win over the Maple Leafs.
"I definitely think the first game got the nerves out a little bit," Donato said. "Being a kid from Boston, it's tough to kind of wrap your head around playing in a playoff game [for the Bruins]. Now having a second opportunity, it's a little more relaxing. I know kind of what to expect and hopefully I'll use that to my benefit."