Rask opted out of the postseason on Aug. 15 to tend to a family emergency, with backup Jaroslav Halak starting each game since.
"Listen, we've said it before: We support Tuukka's decision 100%," Cassidy said Monday, according to Stephen Whyno of The Associated Press.
He added: "It does affect you mentally and it affects how you sort of construct your lineup when you had those back-to-backs."
Rask played in the round robin and two games during Boston's first-round series versus the Carolina Hurricanes. Halak has posted a 4-3 record with a .906 save percentage in the seven games since, and the Bruins face a 3-1 series deficit against the Tampa Bay Lightning with Game 5 scheduled for Monday night.
Dan Vladar made his NHL debut in relief of Halak during a blowout loss in Game 4. Cassidy said the Bruins didn't expect to be dealing with such uncertainty between the pipes when they entered the postseason.
"It's a tough spot for him to go in," Cassidy said. "We saw that when he had to go in in the middle of a game. Then the mental part of it as a group, yeah, you get down. Do you believe you have all the pieces to win? You'd have to ask the players that. I always do."
The Bruins were the NHL's No. 1 team in the regular season with Rask and Halak splitting starting duties.
Rask has made 93 career postseason starts, owning a 51-42 record and a dominant .926 save percentage.