Anthony Beauvillier and Rickard Rakell scored, and Alex Nedeljkovic made 23 saves for the Penguins (24-29-9), who have lost five of six.
“The common theme is we need more purposeful play so we’re harder to play against,” Pittsburgh coach Mike Sullivan said. “I’m not going to single out anything. This is a group effort. I think we could’ve done a better job.
“They got behind us more than we’d like ... but I’m not going to single anybody out. We, as a group, have to be better.”
Pastrnak put Boston ahead 1-0 on its first shot on goal at 1:32 of the first period. He took a stretch pass from Zacha and skated in alone before going backhand to forehand around Nedeljkovic’s left pad.
Pittsburgh has allowed a goal on the first shot in 13 of 62 games this season (21 percent), including in each of the past three. Nedeljkovic also gave up one on the first shot in a 6-1 loss at the Philadelphia Flyers on Tuesday and a 5-4 overtime win at home against Philadelphia on Thursday.
“They all happen different ways,” Nedeljkovic said. “I think for a majority of them, it’s just about starting on time for everybody. Some things are preventable. Some things, they just happen and that’s hockey."
Marchand was injured during a power play at 5:52 when the 36-year-old forward was hit into the boards by Penguins defenseman P.O Joseph. He remained down for several minutes before being helped to the locker room by Pastrnak and a trainer.
Bruins coach Joe Sacco said he expects to have more information Sunday.
"I thought we responded well. I thought we played hard,” Sacco said. “We played hard today. ... Overall, I thought the guys responded very well. We played physical, we played well.”
Mason Lohrei made it 2-0 on the same power play at 6:33 with a snap shot off the right post from the left face-off circle.
Pastrnak was awarded a penalty shot after Erik Karlsson was called for hooking at 3:38 of the third period, but his backhand was turned away by Nedeljkovic.
“’Ned’ was huge,” Penguins forward Bryan Rust said. “I don't think we came out with the game or the energy that we would have liked, and they jumped out to a lead. I think after that lead, he was incredible. He made every save. Made some big saves for us, obviously. The penalty shot was huge, too.”