Mittelstadt is in the first season of a three-year contract he signed with the Avalanche on June 25, 2024, and can become an unrestricted free agent after the 2026-27 season.
The 26-year-old forward has 34 points (11 goals, 23 assists) in 63 games this season.
"I was surprised," Mittelstadt said Saturday about being traded. "I maybe thought something was coming, but it was getting pretty close, so I wasn't sure. But I was excited, of course you want it to work out anywhere you are, so there was some disappointment with that. But I'm very excited to come to such a storied franchise with a great tradition. I know a bunch of guys here as well, so I feel like that really helps the transition with a new team. So, yeah, very excited."
Selected by the Buffalo Sabres in the first round (No. 8) of the 2017 NHL Draft, Mittelstadt has 230 points (77 goals, 153 assists) in 420 regular-season games for the Avalanche and Sabres and nine points (three goals, six assists) in 11 Stanley Cup Playoff games.
Mittelstadt said he hopes to be able to contribute for the Bruins, who traded several players with long histories in Boston on Friday, including Coyle, forward Brad Marchand (to the Florida Panthers) and defenseman Brandon Carlo (Toronto Maple Leafs).
"I think there's a big opportunity, especially with some of the people that are gone," he said. "It's crazy to see some of the people go from here and even from afar I can tell what they've meant. Maybe a little bit of a new beginning and a lot of opportunities, so it's on me to play well.
"I think there's a lot of room for growth. The main thing for me is really finding a consistent game night in and night out. I feel like when I play well I'm really good."
Bruins coach Joe Sacco said the Bruins plan to utilize Mittelstadt's offensive capabilities, particularly on the man advantage.
"He generates offense. ... Good hands, sees the ice very well, can make plays in tight areas," Sacco said. "I think he's obviously going to be counted upon to generate some offense for our group. We're going to use him on the power play tonight and see how that goes."
A third-round pick (No. 76) in the 2024 NHL Draft, Zellers, 18, has 58 points (37 goals, 21 assists) in 40 games with Green Bay of the United States Hockey League.
Boston (28-28-8) is three points behind the Ottawa Senators for the second wild card into the playoffs from the Eastern Conference.
NHL.com independent correspondents Ryan Boulding and Corey Long contributed to this report