He had a knee injury that scuttled his playing career in 1984 and was fired as coach of the Washington Capitals in 2003, but he's always found a way to bounce back.
Given Cassidy's history of resilience, coming back from being down 3-2 to the Toronto Maple Leafs in the best-of-7 Eastern Conference First Round is a realistic goal for the Bruins.
[RELATED: Complete Bruins vs. Maple Leafs series coverage]
Game 6 is at Scotiabank Arena in Toronto on Sunday (3.pm. ET; NBC, CBC, SN, TVAS).
One reason Cassidy has been able to overcome so much is the tutelage of Brian Kilrea, the winningest coach in Canadian junior hockey with 1,193 victories, all with Ottawa of the Ontario Hockey League.
"He's doing a great job," the retired Kilrea said. "His players are playing hard for him. He sees the game one step ahead of most everyone else like he did as a player and he'll make the necessary adjustments.
"I never thought he'd end up being a coach in the National Hockey League. I thought he was destined to be a star player before that freak knee injury occurred. But he stayed in the game and persevered and has become one of the League's best coaches.
"I'm proud of him."
Video: Kilrea on Bruce Cassidy's unique coaching career
Cassidy discovered Kilrea's coaching talents firsthand when he joined Ottawa as a smooth-skating offensive defenseman in 1982. He flourished under Kilrea in his first two seasons, scoring 206 points in 137 games (52 goals, 154 assists) while helping Ottawa win the 1984 Memorial Cup.
"He was going to be a special player in the NHL," Kilrea said. "His vision, his anticipation, his offensive skills -- he would have been like Erik Karlsson in his prime. His head was always up, he was always looking where the puck might be going to."
The Chicago Blackhawks selected Cassidy in the first round (No. 18) in the 1983 NHL Draft. Cassidy made his NHL debut with Chicago at 19 the following March but in the offseason tore the ACL in his left knee in a recreational ball hockey game. He ended up being limited to 36 games (17 points; four goals, 13 assists) in the NHL.
"I never envisioned myself as a coach but the knee injury changed things," Cassidy said. "I wanted to find a way to stay in the game and that was it."
His fourth knee surgery finally convinced to him to retire from Indianapolis of the International Hockey League in 1996. He coached Jacksonville (ECHL), Indianapolis, Trenton (ECHL) and Grand Rapids (American Hockey League) over the next six years.
His break came when he was hired as coach of the Capitals in 2002. He helped the Capitals go 47-45-9-6 before being fired after 107 games.
"He was told not to bench someone and he did," Kilrea said. "Good for him. He did it his way."
Brian Kilrea watching Bruce Cassidy coach the Boston Bruins
The player in question: Jaromir Jagr.
"I learned a lot from that," Cassidy said. "I liked talking hockey with [Jagr]. Some days he did too; some days he didn't. But in no way do I put it on [Jagr] that I lost my job."
Cassidy eventually landed the job coaching Providence, the Bruins' AHL team, in 2008. He was promoted to Bruins coach in 2016, replacing Claude Julien, who led to the Bruins to the Stanley Cup in 2011.
Kilrea sees a lot of Cassidy the player in Cassidy the coach.
"He was one step ahead," Kilreas said. "Certain players have it. [Wayne] Gretzky had it. Like, if you watch replays of Gretzky when he was coming back into his own end, he'd watch which side the puck was going to come on. And if it was chipped out, he'd be there to grab it and lead an odd-man rush. Bruce Cassidy, in the same manner, would read the play before it happened.
"It translates to coaching. He could be behind the bench or I could be behind the bench and sometimes you see a guy who's struggling a little bit and you may have to make a change. Or you might look at the other team and see one of their regulars who is struggling, and you might just say: 'Play off the left side or play off the right side' to force him because he's having trouble. That's the same way with a coach. You see it, recognize it, then do something about it. It's an anticipation.
"Look at his Bruins now. He doesn't have a team with a lot of offensive firepower but he allows his skilled guys to play outside the system. Brad Marchand is an example. He gives him freedom to freelance."
Cassidy said he learned that from Kilrea.
"He would always try to accentuate a player's strength," Cassidy said. "I was an offensive defenseman and he gave me a lot of authority to do what I did best. I try to do the same. (Marchand) is an example. We've had meetings in the summer where I've told him, 'You are too valuable to us to be doing some of that extracurricular stuff you do.' Next thing you know he's thanking me after he got his 100 points.
"I also learned from [Kilrea] that when it goes south, you've got to reel it in. When it comes to your best players, I wouldn't use the term 'call them out,' but you've got to be able in front of the group to say: "This is not acceptable, this is where we draw the line." Then you move on.
"[Kilrea] taught me that too: Be honest with your players. He taught me a lot in the way that I coach."
Cassidy, an Ottawa native, had the chance to deliver that message to Kilrea in person last summer. The two men had a lengthy chat during the Ottawa Valley Hockey Oldtimers Golf Day at the Ottawa Hylands Golf Club on Aug. 10, which was declared Bruce Cassidy Day by Ottawa Mayor Jim Watson.
"I told him to keep doing what he's doing," Kilrea said. "And I let him know I still cheer for all my former players like him.
"Bruce will be fine. He always finds a way."