David Krejci, who had the secondary assist on the play, was honored in a pregame ceremony for playing 1,000 NHL games, which he accomplished on Jan. 16.
"I knew what to expect a little bit, but it was more than I expected," Krejci said. "It was a pretty special moment, having the family there with me. … Just a special moment and I'm very, very thankful that I'm part of this team."
Pastrnak pushed it to 3-1 at 12:10 of the third period, shooting five-hole on a breakaway after receiving a stretch pass off the boards from McAvoy.
Pastrnak has 41 goals this season, trailing Edmonton Oilers forward Connor McDavid (42) for the NHL lead.
"It was a set play," Pastrnak said. "[McAvoy and I] didn't connect the tape-to-tape. He just let it bounce off the boards, and I told him right away, 'That's an unbelievable play.' Heads down to him, great pass."
The Senators were unable to score on a 5-on-3 power play for 1:49 after Bruins forward Craig Smith received a tripping penalty at 9:16 of the first period and Patrice Bergeron got called for hooking 11 seconds later.
"We would love to have that back in a tight game like this," Ottawa coach D.J. Smith said. "But you can't let it affect the rest of the game, which I didn't think it did. I thought we played hard. That's a really good team. We had our chances, I thought we checked really hard. [Mandolese] was really good. It's just at the end of the day, a couple mistakes we made, you know, ended up in our net."
Jake DeBrusk gave the Bruins a 1-0 lead at 15:09 of the first period, putting in the rebound of his own shot over Mandolese's right shoulder. It was DeBrusk's second goal in as many games after missing 17 games with hand and leg injuries.
"It's obviously always a good time when you see the puck go in the net," DeBrusk said. "To get us up in the first there, to get the first goal of the game, it's always [nice getting] the crowd involved, and that's what I wanted to do."