By Matt Kalman, CBS Boston

BOSTON (CBS) — The Bruins’ season opener on Oct. 5 featured five rookies in the lineup. Jake DeBrusk scored his first NHL goal in his debut with a slide of the puck to his forehand and a snap shot into the net.

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The Bruins’ biggest game of the 2017-18 season still featured five rookies in the lineup and again featured DeBrusk putting his speed and hands to great use to score a crucial goal, only this one was the game-winner in a 7-4 win against the Toronto Maple Leafs in Game 7 of the Eastern Conference first round at TD Garden on Wednesday.

Again DeBrusk carried the puck on his hip as his speed increased before he snapped the puck past the goaltender, but this time it was his second goal of the game. And instead of being a wet-nosed rookie trying to fit in, DeBrusk’s a full-fledged permanent member of the Bruins’ top six and a player they count on to help carry them, even during the sludge of a seven-game series against Toronto.

It was a night to celebrate the Bruins advancing beyond the first round for the first time since 2014. But also it was a time to cherish the maturity of some of Boston’s best young players, who’ve reinvigorated the franchise and made the present and future so exciting.

It starts with DeBrusk, who after spending all of his first pro season at the AHL level produced 43 points (16 goals, 27 assists) in 70 games. His first taste of the Stanley Cup Playoffs seemed to electrify him rather than intimidate him, and he tied Pastrnak for the team lead with five goals against the Maple Leafs.

Always the deferential sort, DeBrusk credited center David Krejci for helping him look like a seasoned veteran during the biggest playoff series of his life.

“[Krejci] always wants to make sure I’m focused on the next shift,” DeBrusk said. “I think just the whole season, I think that as the season got on I got more confident in my game. There’s no time to not feel confident now and I just want to do anything I can to help the team. Overall the series went well for myself and went well for the team.”

DeBrusk hogged the majority of the spotlight in the Bruins’ dramatic comeback win (they trailed by one goal three separate times, including 4-3 heading into the third period). However, he wasn’t the only young Bruins buck stomping on the Maple Leafs.

Danton Heinen, back from a one-game press box exile, scored his first NHL playoff goal. Sean Kuraly played just 10:19 in Game 7, but throughout the series he continued to center a fourth line that was every bit a physical momentum changer as it was in the regular season. Defenseman Matt Grzelcyk got banged up early in the series and had his ups and downs with his play, but he never seemed out of place when the intensity dialed up to playoff level.

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And then there’s Charlie McAvoy, who was clearly still trying to find his game during this series after getting just four regular season games to shake off the rest of his knee-injury absence. McAvoy had a hard time rushing the puck, so he started getting rid of it quicker. Unfortunately for him, he often got rid of it to the wrong team. Nonetheless, he kept gutting it up, knowing that at this stage of the season 70 percent of McAvoy is better than 100 percent of anything the Bruins have waiting in reserve. With Adam McQuaid down with an injury for much of the second and third period, McAvoy logged 26:43 of ice time in Game 7 (second to Zdeno Chara’s 28:38). The Calder Trophy snub (he didn’t finish in the top three of the vote) now has more time to get up to speed with a second-round series against the Tampa Bay Lightning rather than head into the offseason on a sour note, individually and team-wise.

The regular season also featured Anders Bjork and Ryan Donato making their marks on the Bruins, who dressed as many as six rookies for some regular-season games. The rookies proved as adept as learning from the Bruins’ best as they were talented with their sticks and skates.

“You can be good, but if you don’t want to learn, obviously, the veteran guys, after a while will say, ‘Hey if this guy doesn’t want it, then we’ll just move along,’” coach Bruce Cassidy said. “So, that’s the other part. You’ve got to be willing to learn, and they have been.”

If the 2017-18 season was about seeing how much youth a team could possess and still succeed, the Bruins answered that question with a 50-win season and the second-best point total in the Eastern Conference. Now in the postseason, that test has continued. And Game 7 was a final exam to see how much character and poise the rookies could show with their backs against the wall.

Sure the Bruins’ veteran core led by Patrice Bergeron, Krejci, Chara and David Backes was going to bring its best performance of the series, and it did. If the rookies wilted under the pressure, it could’ve canceled out everything the veterans did. Instead the rookies met the challenge, even with another member of that veteran core, goaltender Tuukka Rask, having his second-worst game of the series. Not many would’ve given the Bruins strong odds to prevail if Rask let in four goals on 16 shots through two periods. The 18 skaters saved Rask the way he bailed them out in Game 4 with 31 saves behind a Bergeron-less lineup.

No one, not even the Bruins’ brass, expected the dominant regular season that unfolded the past seven months. It was unprecedented to insert so much youth into an NHL lineup and accomplish what the Bruins did in the regular season. Rather than battling until the end of the schedule for a playoff spot the Bruins had their berth wrapped up in late March and kept rolling. That’s why their second-round matchup with the Lightning is a battle of the top two teams in the East, in essence a showdown that should be taking place in the conference finals.

The Bruins can keep the Stanley Cup as their ultimate goal, and they may even soar all the way to June and hoist the chalice. However, even if their run ends in this upcoming round against a more seasoned Lightning team that led the East practically all season, the Bruins have achieved everything anyone could have reasonably expected them to achieve when they opened the season by shocking the defending Western Conference-champion Nashville Predators or after they opened the postseason by outscoring Toronto 12-4 through the first two games of the first-round series.

We know more than half a dozen Bruins with one or two years of pro experience are not only ready to be NHL regulars — they’re capable of thriving in their individual roles. We know in the face of the ultimate pressure, a do-or-die postseason game, they rise to the occasion.

There might be more dramatic Jake DeBrusk goals, better first passes from Charlie McAvoy and perhaps even an unexpected contribution from Ryan Donato in the Bruins’ near future. It will all be gravy because we now know just how bright the Bruins’ future is for this playoff series and beyond.

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Matt Kalman covers the Bruins for and also contributes to and several other media outlets. Follow him on Twitter @MattKalman.