Rask will start for the second straight game after making 26 saves in the win at Buffalo. Jaroslav Halak started five of Boston's previous seven games.
Cassidy called it a tough decision to choose between Rask and Halak, but he wants to give Rask consecutive starts so he can start feeling good about his game again. Rask, 31, is 9-8-2 with a 2.67 goals-against average and .912 save percentage this season. He allowed eight goals in his previous two starts before defeating Buffalo.
"I think it's been not as consistent we'd all agree as we'd like, so this is an opportunity for him to take another step," Cassidy said. "I felt that. I talked to Jaro today about it. He understands. He's had a great first half for us, but that's the decision today and hopefully it plays out well."
The Bruins, though, won't have defenseman Charlie McAvoy, who has a lower-body injury and was ruled out after he missed practice Monday. McAvoy, 21, will miss his second straight game. He hasn't skated since Boston returned from the holiday break last week.