By Joe McDonald (The Athletic)

Brad Stevens had some advice for Bruce Cassidy.

The Celtics coach recommended the book “The Teenage Brain: A Neuroscientist’s Survival Guide to Raising Adolescents and Young Adults” to his Bruins counterpart, not because Cassidy is a father but because he believed it would help the Bruins boss relate to his players.

Cassidy’s immediate thought: “There’s a book I need to read.”

So, he did. Cassidy digested page after page of the New York Times bestseller.

Why?

The Bruins’ roster is loaded with players in their early 20s, and as the NHL has become a younger league because of the salary cap, entry-level contracts and importance of speed, any reading material on how to relate to these new-age players was more than welcomed.

Cassidy’s ability to adapt, teach and coach younger players and an experienced veteran core is one reason he’s now locked in behind Boston’s bench for at least the next four seasons.

On Wednesday, the Bruins announced Cassidy, who has registered a 117-52-22 record in 191 regular-season games along with a trip to the Stanley Cup final last spring, earned a three-year contract extension. The organization does not disclose coaches’ salaries, but it’s believed Cassidy’s new deal falls in the range of $3 million per season. He still has one season remaining on his current deal.

Unless a coach is willing to adapt in today’s game, he will not enjoy longevity with one team because the landscape is continually changing, and Cassidy has shown that rare flexibility.

It’s more of a challenge to have that longevity than it was a decade ago.

Claude Julien lasted 10 years in Boston before Cassidy took over on Feb. 7, 2017. He’s now coaching in Montreal. Joel Quenneville won three Stanley Cup championships during his 11-year tenure (2008-18) with the Chicago Blackhawks. He’s now coaching Florida. Barry Trotz spent 15 seasons (1998-2014) with the Nashville Predators before winning a Stanley Cup with the Washington Capitals in 2018 — and then promptly leaving for the Islanders.

A better example is Tampa Bay Lightning coach Jon Cooper. Like Cassidy, Cooper coached in the AHL before being promoted to the parent club in 2013. He’s coached many of the same players at both levels, and the Lightning have enjoyed tremendous success during Cooper’s seven seasons in Tampa.

The average age of today’s NHL player is 27. In Boston, it’s 26. Coaches are witnessing a drastic change in young players’ motivation. Personalities have changed. Bank accounts are larger for this generation.

So, the question is: How can a coach enjoy longevity and success with one team in today’s game?

“Have quality leaders who believe in his message and support him in the room (because) players fire the coaches now,” said one former NHL coach.

Cassidy arrived with built-in leadership already in place in captain Zdeno Chara, alternate captain Patrice Bergeron and a highly successful veteran core. That group let its new boss know it supported him fully and was available to help however he needed. In fact, the first day Cassidy took over as coach, Bergeron told him: “You’re my coach. I’ll do whatever you need me to do.”

Many were skeptical when the Bruins fired Julien after a decade behind the bench, but Cassidy has re-energized the organization. He’s created a way to bring the Bruins’ brand of size, strength and ability into the modern game with a high-tempo pace.

When asked for his key to success, Cassidy quickly answered: “Ability to teach. The old-school (coach), the guys that lasted a long time obviously had to win, but now there’s more teaching because there are always new kids coming in. It’s a developmental league, and if you don’t have those skills to work with those young guys, I don’t think you have longevity.”

Being able to relate to younger players and knowing they will heed a coach’s message matters now more than ever.

“A coach needs to create different ways to get his message across to players. It may be the same message, but you need to teach it in different ways,” explained an Eastern Conference coach. “This year my assistant coaches built our camp and their voices are going to be heard more than mine.”

Cassidy has figured out how to manage the different personalities in the room and on the ice, which is not an easy task in today’s game. The Bruins have also drafted and developed relatively well under Don Sweeney’s tenure as general manager, and because those younger players are making an impact in Boston, it’s essential for a coach to adapt to the roster under him.

Cassidy’s strengths are evident, and the team’s perennial success is a strong indication of the coach’s ability to communicate well with the players.

“The first thing is to have the process of learning players’ mentality while always communicating,” said a former NHL head coach and current assistant coach. “You have to continue to hold them accountable while making sure your message is crystal clear on what his role and responsibilities are with the team. Young players want the truth, and you have to be honest. Create an environment that these players want to come to the rink and get better — firm but fair. They want to know what’s in it for them. That’s today’s society.”

Time and again, Sweeney talks about Cassidy’s ability to make in-game adjustments. He always preaches the importance of starting on time, and that became the team’s mantra en route to the Stanley Cup final last season.

Moving forward, the coach understands he needs to be a little more patient early in games when things start to go south. Cassidy has been known to get a little too emotional on the bench. In fact, veteran players aren’t shy to tell the coach to settle down a bit, either verbally or with just a look. There’s mutual respect, and Cassidy’s not shy to criticize, teach and coach veteran and younger players. The open line of communication is important to the team’s overall success.

Cassidy plans to let the game develop a little more before making changes when things aren’t going according to script. He’s not going to completely change how he operates, because the results speak for themselves.

Like every coach in the NHL, Cassidy wants to win a Stanley Cup. He came agonizingly close in June, but the Bruins have the resources to return to the final in the near future under Cassidy’s guidance.

His philosophies on coaching and teaching will continue to evolve as he chases the kind of longevity in Boston that his predecessor enjoyed.

“Ask me in five years and I’ll probably have a better answer,” Cassidy said.

(Photo: John Tlumacki / The Boston Globe via Getty Images)