The 19-year-old Lazar gets the experience of playing against the best junior players in the world, Canada gets a polished top-six forward and the Senators hope to benefit down the road from the development their top prospect gets from his second of these tournaments.
"He's going to play a more prominent role," Senators coach Dave Cameron said. "With us he's in the bottom six, playing a safe role. There he's going to be a go-to guy. That's a real world-class tournament on a huge stage, so all those factors are going to make him a better player."
Lazar was Ottawa's fourth-line centre averaging 12:45 a game, second-lowest on the team ahead of only Chris Neil. Wednesday night at the New Jersey Devils, Lazar skated 15 shifts for 8:30, his lowest totals of his rookie season.
Hockey Canada had discussions with the Senators this week about getting Lazar, but the organization didn't find out until Thursday morning that he'd be joining the world junior team. Understandably, those in charge of the team are thrilled to add Lazar to a group already favoured to win gold on home ice.
"He is a big piece of the puzzle," said Scott Salmond, Hockey Canada's vice-president of hockey operations. "He will come in here, hopefully with a lot of confidence and a lot of experience of having been here last year. We expect him to be a leader, and we expect he is going to give our group a boost."
Lazar called it "bittersweet" leaving the Senators in the middle of the season, especially considering the upheaval lately with the firing of Paul MacLean and the hiring of Cameron as coach. But he relishes the idea of becoming that "go-to-guy" at the world juniors.
"The Ottawa Senators, I'm happy to be here, I love to be here and I feel I'm going to have a long, healthy career here with them," said Lazar, who has a goal and six assists this season. "Two weeks it's not going to make that much of a difference."
Having Lazar could make a huge difference for Canada, which already had six returning players: goaltender Zach Fucale, defenceman Josh Morrissey and forwards Sam Reinhart, Connor McDavid, Nic Petan and Frederik Gauthier. Lazar could slide in on Canada's second line on one of McDavid's wings, something he said would be a lot of fun.
Salmond said it was up to coach Benoit Groulx whether Lazar would play wing, as he did in 2014, or line up at centre.
"I see him, imagine him, playing a lot," Salmond said.
That's part of the reason Cameron, who has a history of coaching with Hockey Canada, supported letting Lazar go. Ultimately, the decision rested with Senators general manager Bryan Murray, but he got input from Lazar and Cameron along the way.
Lazar's teammates saw last year what he can bring to a tournament like this when he was a point-a-game performer.
"Curtis brings everything, really," Morrissey said. "He is an all-around player, he brings a lot to our team on the ice with his skills and ability, but off the ice he brings tremendous leadership. He is a hard-working, honest guy."
Before he even skates with this group, Lazar becomes a top candidate along with Reinhart to be named captain. At the very least, the Salmon Arm, B.C., native will be part of Canada's leadership group because he's constantly smiling and can lighten the mood.
"He always brings down the pressure," McDavid said. "He's obviously going to be a huge leader, if not the biggest leader on this team."
Groulx, an assistant last year in Malmo, Sweden, has only good things to say about Lazar on and off the ice.
"He's well-liked by his teammates, respected by his teammates," Groulx said. "He's won everywhere. He's a competitor, everybody acknowledges that. He's such a great young man. I think he'll fit right in with our guys."
The downside of Lazar's addition will be felt by the one extra player who has spent the week at Canada's camp at Meridian Centre in St. Catharines but won't be starting the tournament with the rest of the group Dec. 26 in Montreal. Now three forwards will need to get cut, along with a defenceman, to get down to the 22-man roster.
"That's part of the process," Groulx said. "Having Curtis back, obviously I think it puts a smile on everybody's face on this team. Yeah, it's going to make our decision tougher, but I think it's good for our team to have him back."
Five forwards appear to be on the bubble of making the team: Gauthier, Michael Dal Colle, Jason Dickinson, Rourke Chartier and 17-year-old Lawson Crouse. Though Gauthier is a returning player, the Toronto Maple Leafs prospect doesn't consider his spot safe.
"I think I need to earn it," Gauthier said. "I don't think I got it right now."
Dickinson, who centred the fifth line this week, understands he has to show his value Friday night in Canada's exhibition game against Russia at Air Canada Centre in Toronto. He isn't worried about the Lazar news affecting him mentally.
"It is one of those things that happens in hockey," Dickinson said. "There are going to be guys out there and take your spot, and you just have to battle through it."
There's little doubt that Canada is a better team with Lazar than without. He's the second full-time NHL player to join after the New York Rangers released Anthony Duclair to play.
"It obviously makes us a lot deeper," McDavid said of adding Lazar. "There's so many good players throughout this country at this age. Obviously he's one of the best. He's going to help all around."
Lazar will meet up with the team in Toronto but won't play in Friday's exhibition game. He's hoping to "put on a show for the Sens fans" Sunday when Canada plays Sweden in an exhibition game at Canadian Tire Centre.
Obviously the long-term goal is the games that matter and Lazar trying to help Canada win its first world junior gold medal since 2009.
"We're on home soil here and the expectation is for us to win gold," Lazar said. "My junior resume, it's pretty complete except for a medal at the world juniors. It's a big opportunity here and I like the team that they have in place so far and I'm going to go and contribute and hopefully I can check that one off the list."
With files from Lisa Wallace in Ottawa
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Note to readers: This is a corrected story. A previous version said there were six returning players.