The Bruins looked to be in deep trouble in their first round series against the Toronto Maple Leafs.

Andrew Peeke had shored up the Bruins back end with his rugged, in-your-face style of play when he was acquirred form the Columbus Blue Jackets aat the trade deadline. But he got hurt in Game 2, which the B’s lost in the Garden.

The B’s decided it was time to throw rookie Mason Lohrei into the deep end. They were not disappointed. The kid was a natural-born swimmer.

The highly-touted Lohrei had looked very much like rookie in his 41 NHL games – terrific one shift, not so much the next. But in the playoffs, he met the moment. He was consistently good on breakouts and, occasionally, excellent in the offensive zone. But most of all, he wasn’t afraid, despite the stakes.

It was just the first step – albeit a big one – of what could be a standout career.

“His foundation is great and he works every single day. I can’t wait to see the strides that he makes,” said Charlie McAvoy on Sunday. “I keep telling him the sky’s the limit for him and I see where he can go. It’s really going to be massive for us to see the strides that he can take this summer. Watching him come into his own, I’m excited that I’m going to ge to be here for that and help him in any way that I can.”

McAvoy, wo had een an off-and-on partner for the rookie, sees Lohrei as just a part of promising undercurrent of young blood that should catapult the organization forward.

“Someone like him, and what we got out of (Mattt Poitras, John Beecher, Justin Brazeau), all the guys that stepped in and you see the depth within the organization start to show itself,” said McAvoy. “I think that really means so much. You’re seeing these foundational pieces tat are being laid for something special tat I think we’re building here.”

Lohrei’s accelerated development should be a boon for GM Don Sweeney as he continues to navigate a crucial Year 2 of his reboot. The B’s have approximately $21 million in cap space which could grow if Linus Ullmark is dealt. Sweeney needs to bolster the back end, but he doesn’t have to search for a top-of-the-market top-four defenseman with McAvoy and Brandon Carlo on the right side and Lohrei and Hampus Lindholm on the left. They could use an established third pair left D who potentially could bump up in the lineup in close-out situations, but that shouldn’t break the piggy bank.

The way the season finished – both from a positive personal standpoint and disappointing team aspect – has sent Lohrei into the offseason with a full head of steam.

“It’s been incredible,” said Lohrei, whose stick skills are elite. “I was just telling my parents that it was the most fun I’ve had playing hockey, ever, this playoff run we had. Obviously being able to come in and jump into a role and run with it is huge for confidence and going into the summer is definitely giving me a lot of motivation to stay at it.”

What did he learn about himself in the playoffs?

“Just having that confidence to go out there and make plays, (learning) the game management, when to take risks, when not to,” said Lohrei, who had a goal and three assists in 11 playoff games. “Plus, it’s hard, and being able to go out there and hold my own and try and be physical and box out in front of the net and win battles (was a positive). It’s obviously stuff I need to continue to work on but it’s good see that I can go out and compete.”

How much bigger and stronger he can get remains to be seen. He’s 23, not a teenager. But he is at the start of the is pro career and now he has a better idea of where and how he needs to improve.

“I always want to work on getting stronger, the first three steps, speed. And then obviously keep working on the little things — skill, edges, skating, all that stuff, kind of touch everything. But a big emphasis on getting stronger,” said Lohrei, adding that initial burst is important at this level, too.

“You see some guys it comes naturally to, like Beecher. That’s just God-given. But I’ll go home, (use) plyometrics and strength (training). It’s something that I’ve worked on a lot and gotten better at, so it’s good to see the progression. But I want to keep that going and to be able to have the kind of separating first three steps. It’s so valuable out there. There’s not much time and space to be able to create that for yourself.”

Lohrei has made the first part of his dream come true. He is an NHL player, to be sure. But the bar he has set for himself is so much higher.

“I want to be great. I want to win the Stanley Cup. I want to be a top defenseman in the league,” said Lohrei. “It’s just the way I think. I just have to keep working and showing up every day.”

He said that without a trace of cockiness, just the confidence of someone who has glimpsed what it takes to succeed at the highest levels of the NHL. And with the way Lohrei rose to the occasion in his first taste of the playoffs, it would be wise not to bet against him.