By Matt Kalman, CBS Boston

BOSTON (CBS) — You can’t miss the irony that Bruins forward David Pastrnak took a holding penalty while backchecking in overtime in Game 6 of the Eastern Conference first-round series against Ottawa at TD Garden on Sunday.

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The penalty cost the Bruins, who allowed Clarke MacArthur to score the game- and series-winning goal 36 seconds later.

The Bruins lost the series 4-2, with every game decided by one goal, including four that went to overtime.

But back to Pastrnak, he had a breakout campaign in his third NHL season with 70 points (34 goals, 36 assists). He became part of the best line in the NHL with Patrice Bergeron and Brad Marchand, and helped get David Krejci’s juices flowing again on the second line after Bruce Cassidy replaced Claude Julien as coach in February.

“He’s backchecking, which is a part of his game, which he’s really focused on playing the 200-foot game, especially in the playoffs,” forward David Backes said. “He definitely shouldn’t hang his head.”

Ever since general manager Don Sweeney took over as general manager, the Bruins have been boasting about their future while trying to keep some focus on the present. They got what they wanted this year with a late-season six-game winning streak that wrapped up the organization’s first postseason berth in three years. And they got a glimpse during the regular season and playoffs at what they have in terms of personnel for the years ahead.

Young players are going to make mistakes, and the 20-year-old Pastrnak was not immune. One could argue he was preventing a 2-on-1, but defenseman Zdeno Chara was back helping to protect the Bruins’ net when Pastrnak pulled down MacArthur during Erik Karlsson’s rush with the puck into the Boston zone. The execution by Pastrnak was poor, but no one could fault the effort.

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Critics will pick on Pastrnak for following up his prolific regular season with two goals and two assists in six playoff games. He was obviously fighting the puck and having a hard time getting space once the Senators keyed in on him, but Pastrnak got his chances and wasn’t a liability without the puck. He’s come a long way since he was a teenaged rookie learning the ropes from Julien.

Rookie defenseman Charlie McAvoy had a handful of dicey scenarios in overtime as well. None of them cost the Bruins like Pastrnak’s penalty, but the Senators had clearly finally figured out some of the 19-year-old’s tendencies. Nonetheless, McAvoy averaged more than 26 minutes of ice time in his first six NHL games, had six assists, and gave the Bruins a reason to believe life after Chara will be fine.

Sean Kuraly made his mark with two goals in Game 5 and proved he could be a key depth forward the next time Boston hits the ice. Joe Morrow and Colin Miller earned their keep on the back end after the injuries to Adam McQuaid, Torey Krug and Brandon Carlo forced Cassidy and Sweeney to dig down the depth chart. Morrow and Miller at least proved they could be valuable in trades, if not as third-pair defensemen in Boston.

And although 29-year-old Kevan Miller isn’t part of the young core moving forward, he established himself as part of the fabric of the Bruins. The younger players who’ve already arrived in Boston, and the ones on their way (Jeremy Lauzon, Zach Senyshyn, etc.) are going to need players to show them the ropes and stick up for them. Miller added an offensive zeal to his solid defensive game after Cassidy took the reins, without losing an ounce of his physicality.

The Senators’ best players played better than the Bruins’ best players for stretches of the six game series, but more importantly, depth won out. Ottawa had a handful of injuries to players down its depth chart, while the Bruins were living without three of their top four defensemen. Ottawa had a more balanced attack. The Bruins were also breaking in 11 players who had never played in the playoffs in the NHL. It was too much to overcome in the present, but in the future, this experience will pay off.

McAvoy will keep learning to move the puck with poise and Pastrnak will keep backchecking. And if Sweeney’s allowed to carry on with his plan there will be fewer playoff disappointments and more joyous celebrations in the near future.

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