Frederic gave Boston a 2-1 lead at 7:07 of the third period with a wrist shot past a screen from Pastrnak.
"It's almost like a four-point swing at all times when you're playing," Frederic said. "Pittsburgh's ahead of us [in the MassMutual East Division]. So it's a huge two points, two points they can't get in a game. I just think as a group, the way the last couple games have been going, hopefully we can build off of that."
Pastrnak put the Bruins ahead 1-0 on a power play at 3:20 of the first. After taking a cross-ice pass from Brad Marchand in stride, Pastrnak scored on a backhand through the five-hole.
Vladar denied Colton Sceviour a tying goal by lunging to his right for a stick save that kept a wrist shot from going into an open net at 9:17 of the first. Tanev tied it 1-1 at 15:22 when he scored on a wrist shot from the right post after Evan Rodrigues tipped a shot from Kasperi Kapanen.
Though Boston allowed Tanev to score later in the period, Bruins coach Bruce Cassidy said the save on Sceviour was critical.
"It certainly gave the group confidence," Cassidy said. "The whole bench got up. When you make a save like that, it's bound to lift the guys. What it does for him? I don't know. I don't know him that well, but I assume it makes him feel good. It certainly gave our guys a lift and it gave their team something to think about."
The Bruins failed to score after Tanev was given a five-minute major penalty and game misconduct for boarding Tinordi at 12:57 of the second period, leaving the Penguins with 10 forwards for the rest of the game. Tinordi was helped off the ice and left the game with an upper-body injury.
"We're going to do the best we can with what we have," Sullivan said. "That's what we did tonight. We had a short bench for quite a long time tonight. We moved people around to try to keep everybody involved.
"When your bench gets that short for that long a duration of the game, we're trying to spread the workload as best we can. I thought the players played extremely hard tonight."
NOTES: Malkin's 12-game point streak (four goals, eight assists) ended. He had a goal and an assist Monday to reach 1,100 NHL points. … Penguins forward Teddy Blueger was a late scratch with an upper-body injury that will have him out longer term. … Vladar's only other NHL game was for the Bruins against the Tampa Bay Lightning in Game 3 of the Eastern Conference Second Round on Aug. 26, 2020. He relieved Jaroslav Halak and made 12 saves on 15 shots in a 7-1 loss. … Cassidy did not have an update on Tinordi after the game but said his chances of playing against the Buffalo Sabres on Thursday are "not great." … Pittsburgh is third in the division; Boston is fourth.