Boston got off to a decent start behind Grzelcyk's power-play marker just 6:47 into the contest and opened up a 9-1 advantage in shots during the opening minutes. But things turned quickly for the Bruins late in the frame as Pittsburgh struck for two goals in 1:43 - with both tallies likely ones that Jaroslav Halak (23 saves) would like back.
The first came with 3:09 remaining in the first when an Evan Rodrigues shot trickled through Halak to tie the game. Sidney Crosby followed shortly thereafter with a sharp angle shot from the left corner that sneaked in between Halak and the post.
"He wasn't good in the first period," Cassidy said of Halak. "He's played a lot of hockey for us lately, a lot of good hockey. You have to push back. He's bailed us out lots of times…2-1 game, you've played just as good as they have, if not better, so you keep pushing.
"Where I'm disappointed is we got away from our game in the second period, in terms of playing forward against a team that can really skate."
Pittsburgh added another goal midway through the second when Evgeni Malkin struck on the power play to make it 3-1 Penguins, a dagger of sorts for a Bruins team that has struggled mightily to generate offense in recent games.
"It does feel like every mistake is going in our net," said Brad Marchand. "That just means we've got to clean up our game a little bit. When things aren't going well and you're not scoring the way you want to, you've got to play almost mistake-free hockey.
"We had a much better effort tonight. If we play like that again [on Tuesday], we'lll have a much better result."