Bruins’ David Pastrnak bumped off the No. 1 line

David Pastrnak entered Tuesday with just one goal in his last nine games.File/PAUL SANCYA/ASSOCIATED PRESS

In Tuesday’s first period, when the Bruins went on the power play in search of the tying goal, David Pastrnak tried to carry the puck into the offensive zone.

Mark Giordano squashed that entry.

The Calgary captain thumped Pastrnak so hard that the right wing lost his stick. On the following wave, Pastrnak and Giordano clashed once more. This time, Pastrnak took exception to Giordano’s check by slashing the defenseman and ending the Bruins’ power play.

The penalty was one reason Bruce Cassidy called for a change.

For the final two periods, David Backes took Pastrnak’s spot on the No. 1 line during even-strength play. The switch worked. The Bruins scored four straight goals and claimed a 5-2 come-from-behind win over the Flames. Backes and Brad Marchand assisted on Patrice Bergeron’s second goal.

“Stroke of genius, huh?” Cassidy cracked.

The Bruins coach uncovered two solutions with one adjustment. The top line improved with Backes’s meaty presence. But Cassidy also sent a message to Pastrnak: No jobs are safe when selfishness, soft play, and underwhelming performance are submitted.

“You’re going to have tough matchups come April and May,” Cassidy noted. “If we’re fortunate enough to be playing well and playing at that time of the year, that’s what he’s going to see. He’s going to have to grow from the experience he got last year. So there was a little bit of [message-sending], for sure.

“I love David’s passion for the game, his willingness to compete. We just have to remind him every once in a while how to compete, how to manage the puck, and how to best help the team.”

For most of the season, Marchand, Bergeron, and Pastrnak have formed the league’s best three-zone line. Of late, however, Pastrnak has gone quiet. The right wing has one goal in his last 10 games.

On Tuesday, Pastrnak had a good chance off a Danton Heinen feed that David Rittich kicked out in the second period. Two nights earlier, Eddie Lack got in front of Pastrnak’s short-range backhander.

But hot goalies are not the only reason Pastrnak’s production has fallen off. He has not been able to get open to put his strengths to use. Pastrnak has landed only seven shots in the last five games. When he’s on, like he was in an eight-shot barrage against Montreal Jan. 20, Pastrnak is a puck-pounding machine.

Pastrnak’s one-timer, usually his moneymaker, has gone missing.

On the power play, the Bruins’ best goal-scoring option is for either Torey Krug or Ryan Spooner to set up Pastrnak for a blast from the left circle. Pastrnak owns the team’s best one-timer. He does not require much time nor space to launch the puck into orbit behind a helpless goalie.

In practice, it is much harder to do.

Pastrnak’s last goal, which he netted against Toronto on Feb. 3, was a power-play strike. But it was scored off the rush past Frederik Andersen, not of the one-time variety. Opponents are not granting Pastrnak many windows in which to send such pucks on goal. Four years into his career, Pastrnak has developed into the kind of goal scorer that penalty-killing coaches devise game plans to deny.

It is not just Pastrnak’s 19 career power-play goals that are causing caution for opponents. Bergeron’s mastery of the bumper position between the circles is prompting penalty-killers to lessen two options for the price of one.

“When they take away the bumper and take away Bergy, they’re also taking away Pasta,” Krug said. “Because the puck can’t get up to me. They’re trying to keep Spooner down low. They’re also trying to take away Bergy and Pasta at the same time.”

It’s what any clear-thinking PK coach would instruct. Only T.J. Oshie can play the bumper as well as Bergeron. Even while facing the shorthanded heat, Bergeron has canned a team-high nine power-play goals. You know how most of those take place: with Bergeron, toes pointed at the net, rapidly snapping pucks from just inside the right dot even when checkers are trying to close.

Bergeron is just as dangerous when he’s distributing pucks as the bumper instead of ripping them on goal. He is the safety valve for Krug and Spooner, quick to bump pucks right back at them or distributing them elsewhere around the rotation.

So if Spooner, for example, controls the puck on the right side of the setup, his options are limited and not as threatening if the killers swarm Bergeron. It usually forces Spooner to carry the puck lower down the right-side wall, where limited space makes working conditions more favorable for the penalty kill.

If Spooner runs out of room, Marchand, usually his closest available teammate at the right-side goal line, doesn’t have much space to play with the puck either. Krug has to rotate from the top to lower on the right side as support. So if Spooner passes to Krug, the defenseman is left with a long and low-percentage cross-ice feed to Pastrnak. The killers have done their job.

“The other teams know that’s one of the strengths of our team,” Krug said. “We’re scoring power-play goals when the puck is rotated up top, then it’s going over to Pasta. Then it’s on net. Then they’re scrambling. So then we hit a seam and we score a goal.

“Now teams know that to slow us down, they have to create battles that are more beneficial for them. Four-on-four below the tops of the circles is probably something that’s better for them.”

Pastrnak’s absence of power-play production may have affected the rest of his game. So Cassidy grabbed Pastrnak by the figurative collar with a coach’s go-to move: a demotion.

Pastrnak, while riding with Heinen and Riley Nash on the third line, played like a player annoyed by the move. He skated hard, pushed back physically, and played at a higher tempo.

“He knew he took a bad penalty,” Cassidy said of the slash. “He came out of the box ready to go in the second period. He was physical. He was winning pucks. Nice play, Heinen found him, got his shot on net. He wanted to give back. He knew he messed up. That’s the growth you like to see him.

“You don’t like to see a guy go pouting, sit at the end of the bench, and not respond. That’s maturity. He responded the right way, played hard, and helped us win a hockey game.”