David Pastrnak celebrates after his third goal of the game Saturday night against the New York Islanders. The Bruins won 5-2 in Game 1 of the East Division finals. Elise Amendola/Associated Press

BOSTON — David Pastrnak flinched once, then twice, while he drifted into position for the perfect wrist shot that beat Ilya Sorokin for goal No. 1.

His second goal came in a flash as he found a quickly closing shooting window.

To complete his hat trick, Pastrnak again waited out the defense, bringing the puck over the blue line, then dipping it toward his backhand before stickhandling into the middle of the ice and wristing it past the Islanders goalie to clinch a Game 1 victory for the Boston Bruins.

“The fact that he waited them out does show that he’s gaining some of his confidence back, and a bit of his swagger,” Bruins Coach Bruce Cassidy said Sunday, a day after Boston won 5-2 in a rejuvenated TD Garden to open the East Division finals. “So that’s good for us. When goal scorers get hot, that’s what happens a lot of the times.”

Islanders Coach Barry Trotz said he didn’t blame his goaltender for the loss, but he declined to commit to another start for Sorokin in Game 2 on Monday night. Semyon Varlamov started 35 games in the regular season to Sorokin’s 21 and averaged 2.04 goals allowed to Sorokin’s 2.17.

“I have a couple more hours to mull it over,” Trotz said Sunday.

Pastrnak tied for the NHL lead with 48 goals during the 2019-20 season, then dipped to 20 in 48 games this year. He had just four goals in the last five weeks of the season and started the first-round series against Washington with three straight goalless games.

But Pastrnak now has three multi-point games in a row – including his second career postseason hat trick.

Only Hall of Famers Phil Esposito (4), Cam Neely (3) and Johnny Bucyk (3) have more three-goal games in the postseason for the Bruins, and this one was perfectly timed for the shower of hats from Boston’s first full-capacity crowd since the start of the pandemic.

“It’s a tradition. It’s always fun – especially if it’s your team that scored a hat trick,” Pastrnak said. “You could feel the energy already this morning. We were all excited. On the warmup it felt like 22 players playing their first NHL game. Everybody looking around and so many people. It’s definitely a different game with the fans, and obviously a lot of fun today.”

With Saturday’s outburst, Pastrnak is second to Colorado’s Nathan MacKinnon with five goals this postseason. He also has four assists.

Asked what his team wasn’t doing against Boston’s top line of Pastrnak, Patrice Bergeron and Brad Marchand, Trotz said, “Keeping them off the scoreboard.”

“We’ve got to challenge them a little bit more,” he said. “We backed off. They’re going to make plays through you.”

And that’s what Pastrnak did all night.

Cassidy said the 25-year-old Czech’s patience helped him find openings that might not have been there for a player who was feeling the pressure of a scoring slump.

“I think earlier this year he was trying to jam that in earlier than he needed to,” Cassidy said. “Some of that was because he hadn’t scored in a while. So you press, and it’s human nature, you want to get it in there before the goalie’s set.

“Nothing’s going in and it’s like, ‘OK, I’ve got to get it off even quicker. But sometimes it’s the opposite: Take your time a little bit more, get your head up, and try to get it where you need to get it.”

Pastrnak’s third goal came with 4:10 left after the teams spent the first 55 minutes within a goal of each other. Taylor Hall added an empty-netter in the final 90 seconds.

That was a relief for the crowd, which was listed at 17,400 – the largest to see an NHL game in more than a year.

“The energy, the atmosphere was everything we expected and more,” Bergeron said. “To say that we’ve missed them is an understatement. I think you appreciate it even more when the fans kind of are taken away from the game a little bit for quite some time and you have to play without them. … Special, special night. It was good to have them and good to have the win.”

Cassidy said that he would see how Craig Smith feels before Game 2. The Bruins right wing missed the third period Saturday because of an unspecified injury after a leg-on-leg collision with Islanders forward Cal Clutterbuck.

Defenseman Matt Grzelcyk left the game for a period but was able to return, and Cassidy said he expected him to be available on Monday night. Defenseman Kevan Miller has missed Boston’s last two games and isn’t ready to return.