Canada left one spot open for a forward when it named its roster for the World Juniors on Dec. 13 with the hope that an NHL team would release a player for the tournament before the NHL holiday roster freeze goes into effect Wednesday.
Poitras and Buffalo Sabres forward Zach Benson were the leading candidates.
"This is quite an accomplishment for all young hockey players, a chance to represent their country at one of the biggest stages," Sweeney said. "Now, it's unique in the fact that he's playing in the NHL. So that's the first thing that probably goes through anybody's mind, what's the trade-off? But there really is no downside for him to go and represent and be a leader on that team and hopefully accomplish the ultimate goal as all young players certainly playing hockey get a chance to try to accomplish. We're excited for Matty and he'll represent the Boston Bruins, himself, his family and Team Canada really well."
Poitras made the Bruins roster out of training camp and had five points (three goals, two assists) in his first six games.
He had two assists in his past three games, including in Boston's 2-1 overtime loss to the New York Rangers on Saturday, when he played 14:44, his most ice time in six games.
"This is what gets him to a chance to mentally and physically to have a little bit of a reset and go play against your peer group," Sweeney said. "Now, again, it's high expectations, high leverage, pressured situations that he's now been thrust into at the highest level you could possibly play at. He should handle that well. He should take those things that happen in our locker room and impart them on his new teammates and lead from the front. That's what we're hoping for. And he'll come back and reinsert himself with us."
Canada opens the World Juniors against Finland on Dec. 26 (8:30 a.m. ET; NHLN, TSN). The tournament, which Canada has won the past two years, runs through Jan. 5.
"You never want to leave the NHL, but I'm excited to get over there and play for my country," Poitras said. "I'll never complain about that. It's a dream come true."