Poitras, a 19-year-old forward selected in the second round (No. 54) by Boston in the 2022 NHL Draft, scored on a rebound off Jake DeBrusk's wrist shot to give the Bruins a 2-1 lead at 10:20 of the third period.
"If you're going to produce in this league, you have to be willing to play inside the dots and that's what I love about him," Boston coach Jim Montgomery said of Poitras. "Whether it's one-on-one battles in the corners or getting to the hard areas, he's willing to go to areas where you're going to have success."
Brad Marchand scored into an empty net with 2:34 remaining for the 3-1 final.
Marchand has seven points (four goals, three assists) in five games this season.
"Three (games) in four (days) on the road with travel against an Anaheim Duck team that played really physical, played with a lot of emotion and they had us on our heels in the first there, physically," Montgomery said. "I think we got a little bit better as the game continued to go on, but it was a big win for us."
The Ducks went on a power play 29 seconds into the game when Marchand was called for cross-checking defenseman Urho Vaakanainen, who was making his season debut.
Boston's best scoring opportunity through the first two periods came 41 seconds later on a shorthanded breakaway by DeBrusk, who hit the inside of the left post.
Troy Terry nearly had a breakaway coming out of the penalty box with about 3 1/2 minutes left in the second period, but he was unable to settle a stretch pass from defenseman Ilya Lyubushkin, and Ullmark swept the puck aside.
"We've realized that playing hard and doing the right things, we're close, but we're not there yet," Ducks forward Ryan Strome said. "So we've got to dig a little deeper, find a way to get that timely play."
McTavish gave the Ducks a 1-0 lead at 5:05 of the third period. He passed to Strome off the rush and his wrist shot from the right circle was saved, but McTavish pushed the rebound off the left post and across the goal line.
