He is that, in some ways, even so. He has spent this season molding rookies into key contributors for one of the NHL's best teams, with six of them playing at least 30 games for the Bruins, in addition to 21-year-old forward Ryan Donato, who signed March 18 out of Harvard University and played 12 games. He has coached high-end prospects like rookie defenseman Charlie McAvoy, 20, and grinders like rookie forward Sean Kuraly, 25.
Cassidy has also taught rookie forwards Danton Heinen (22), Jake DeBrusk (21) and Anders Bjork (21), and rookie defenseman Matt Grzelcyk (24). He has enabled each to find his path in the NHL, and quickly, taking a team that was expected to finish in the middle of the pack and creating a juggernaut that finished second in the Atlantic Division and Eastern Conference this season at 50-20-12, one point behind the Tampa Bay Lightning. Boston will host the Toronto Maple Leafs in Game 1 of the Eastern Conference First Round at TD Garden on Thursday (NBCSN, CBC, TVAS, NHL.TV).
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It was part of why general manager Don Sweeney fired coach Claude Julien on Feb. 7, 2017, and replaced him with Cassidy, an assistant. Cassidy had spent the previous eight seasons with Providence of the American Hockey League, the last five as coach.
Sweeney knew he might have youth surging into the roster. He said he believed Cassidy and the assistant coaches he'd bring on would handle that.
"I'm used to younger guys," said Cassidy, who helped Boston go 18-8-1 in its final 27 games last season after it went 26-23-6 under Julien. "It's my background. I have a lot of time for them, and some of it is because when I was a younger player I never quite made it. A lot of that had to do with my health and talent, obviously, but I always thought when I was a young guy I was a little bit lost, my personality. I wasn't a gregarious type. So some of these kids, I find you don't want them to slip between the cracks."
He has spent this season doing exactly that, helping the rookies blend nearly seamlessly with the veterans. Those six rookies, plus Donato, were responsible for 172 points. Heinen finished fifth on the Bruins with 47 points (16 goals, 31 assists), DeBrusk was seventh with 43 (16 goals, 27 assists), and McAvoy 10th with 32 (seven goals, 25 assists). McAvoy was second on Boston in average time on ice (22:08) behind partner Zdeno Chara (22:54).
The rookies have been crucial.
Cassidy's work with the veterans is still evolving, by his own admission, in his second season back in the NHL after an initial foray as the head coach of the Washington Capitals a decade and a half ago, from 2002-2004.
But his work with the rookies has been his masterstroke. He has utilized his passion and his perception to push them forward -- and the Bruins with them.
"It's like parenting," Cassidy said. "I enjoy watching them grow."
Video: The crew previews the Bruins-Maple Leafs series
Torey Krug has seen both sides. He played under Cassidy in Providence as a 21-year-old first-year professional after signing with the Bruins on March 25, 2012, as a free agent out of Michigan State, and is playing under him again as a veteran in his seventh NHL season.
The two are alike, in that Krug plays the way Cassidy did when the coach was an offensive-minded defenseman in the 1980s. Cassidy's NHL career spanned six seasons but only 36 games, all with the Chicago Blackhawks, with knee injuries taking their toll. Cassidy said he knew when Krug arrived that they would connect. But that connection has not always been smooth.
"He was really tough on me," Krug said.
It made Krug's transition difficult at the time. He didn't quite understand what Cassidy was getting at, what he was pushing him toward. He had always been the best player, always played the way that suited him, as a puck-moving defenseman. It was what had gotten him through Michigan State and into the professional ranks.
So at times he chafed at the ways Cassidy tried to instruct him.
"When you show up to this level and things are a little bit harder for you and you have someone telling you how you should be playing or how you should approach something, you don't want to trust them at first because you've always done it a certain way," Krug said. "I think the pride gets in the way big time. Once you've been able to swallow that and put it behind you and really have a collaborating effort with the coach, it helps big-time."
It took until January 2013, during Krug's lone AHL season, for him to come to terms with Cassidy and his methods. Krug saw how easy it was to get offended, to take it the wrong way when the coaching staff tried to teach him. It made him nervous, unsure and insecure.
"I think more than anything it's a mentality thing, realizing that, hey, my coach is on my side and he's not against me," Krug said. "That's a big part of it. … Bruce and I really butted heads and got in a lot of battles, then you realize that he just sees the potential in myself and realizes the plays that I should be making on a nightly basis and if I'm not doing them, he's going to let me know about it. It took me a while to figure that out."
He did, and it changed his outlook. It allowed him to see that Cassidy was rooting for him, hoping to see Krug succeed where Cassidy did not, pushing him to be his best on the ice.
It's something that Krug said he sees, even now. He sees Cassidy doing the same with the current crop of younger players, nudging them to become better, to lift the Bruins.
"He's definitely very hard on younger guys, but it's in the right tone and it's in the right context," Krug said. "As long as the player isn't taking things too personally, then he'll be able to get better. Now working with a veteran group he's more understanding that guys have a longer leash in terms of things you can do on the ice and making mistakes, but also he's very hard on the older guys to maintain that level of standard that we play by.
"He's a very demanding coach, but you respect that out of him."
Cassidy doesn't often talk about what happened with the Capitals, who fired him 28 games into his second season. He was young when he got his first shot at being an NHL coach, 37, not much older than some of his veteran players, and younger than Chara (41) and forward Brian Gionta (39) are now.
"The team I inherited was veterans, guys that had been around, from a lot of different countries, so you had a lot of -- not that that matters -- just had a lot of different personalities," said Cassidy, who went 47-47-7 with nine ties in Washington. "I don't think it was ideal for me."
This time it has been better, more suited for him, with Boston trying to get younger.
"It's a little bit easier to walk into the room for me personally," Cassidy said.
He had worked with some of the players before, like Krug, forwards Noel Acciari, David Pastrnak and Ryan Spooner (traded to the New York Rangers on Feb. 25), and defenseman Kevan Miller. But even with those he hadn't seen before, he related better.
He found his voice.
And, by all accounts, he uses it.
"I think [he has] that ability to communicate across the younger group of kids, get his message across - colorful at times and very, very direct at times," Sweeney said. "They know there's a teaching moment that happened there and then, OK, we've moved past, how do I get better today, how do I apply that?"
It's part of how Cassidy prefers to work. He would rather coach in the moment, get out any frustration or any teaching opportunities, and move on. That can make it difficult sometimes for those who take criticisms to heart, who haven't quite made the mental leap that Krug eventually did, understanding that the toughness was out of respect.
He has tolerance for them. Patience.
The other part? The veterans? He's still learning, not entirely comfortable with it. But he said it will come with time, with more listening and more comprehension.
"He's a newer coach up here and he's open for anything, suggestions from guys," Acciari said. "He's always asking guys, 'Do you want something different? Let me know and we'll talk about it.' He's easy like that.
"I think that helps a team out because, yeah, he's going to teach us everything he can along the way, but he also wants to learn too."
That is clear. When Cassidy was asked about the influence he or any coach can have on young players, on their ultimate successes or failures, he hesitated.
"I don't know yet," he said. "I'm still learning that, to be honest with you."
He pointed to Brandon Carlo, the 21-year-old defenseman in his second NHL season who was excellent (16 points; six goals, 10 assists in 82 games) as a rookie under the tutelage of Cassidy and Chara. This season did not go as well, with Carlo (six assists in 76 games) taking a step back at times before fracturing his ankle against the Florida Panthers on March 31 (he is out for the season).
"I've moved on to Charlie and DeBrusk and haven't had much time for Brandon," Cassidy said. "I feel bad because I think there's parts of his game that we worked on last year, the offensive part of it, that aren't quite there again this year and I want to go back to it, but it's only in bits and pieces and you're working with everybody."
It is frustrating when that happens, for the player and coach.
But these are the parts of his job that he is still working through. Cassidy, like all coaches, is trying to determine exactly the effect he can have on young players, and whether any coach can make a lasting difference.
"I think you definitely have a short-term impact," Lightning coach Jon Cooper said. "It's more difficult to have a long-term impact because whether you're giving the Knute Rockne speech or whatever tricks you're pulling out of the bag, eventually you have to train them for long-term success."
That's the ultimate goal, finding long-term success for the player and for Boston and for Cassidy himself.
They can all see the possibilities, given where the Bruins are after finishing third in the Atlantic and seventh in the conference last season at 44-31-7, losing to the Ottawa Senators in the first round. It has been a surprising turn of events for a team so reliant on rookies.
But that's what Cassidy has done. It makes sense. It's in his DNA.
"A ready-made player, I don't want to say there's no challenge, but there's a challenge with the younger guys," he said. "I think if I didn't end up a hockey coach, I would have ended up some sort of teacher because I actually enjoy that part: the challenge of watching a kid get better."