It may be because there had been doubts the Longueuil, Que., native would make the team — despite lighting up the Quebec Major Junior Hockey League this season — because of his questionable defensive play.
No one doubted his ability to put the puck in the net, as Mantha demonstrated with a hat trick in Canada's 7-2 win over Germany on Thursday.
"It's a great feeling," the lanky, six-foot-four right winger said. "For me, personally, I had to get a big game going and I think that's what I did."
It helped that Mantha was on a line with perhaps Canada's most dynamic playmaker in Jonathan Drouin, who assisted on all three of his goals — two on power plays and one at even strength.
But Mantha, the grandson of former NHL forward Andre Pronovost, scores wherever he plays.
He had 35 goals in only 32 games with the Val d'Or Foreurs before joining Team Canada two weeks ago for the run-up to the world juniors. He has 85 goals in 99 games over the last season and a half.
Coach Brent Sutter said what clinched his spot was that ability to put the puck in the net, which there is not a lot of on this squad compared to past world junior teams. Sutter hopes a crash course in 200-foot hockey will make the Detroit Red Wings prospect a more complete player as the tournament goes on.
"He can score goals, but there are other aspects of his game he needs to continue to work on," said Sutter. "Things I've talked to him about one on one.
"It's simple. It's being a competitor in all three zones and making sure he's responsible in all three zones. Not just for this tournament, but for him to be a good pro. Especially with the organization he'll go to in Detroit. That's what they expect there. You can't just play the offensive side of the puck."
Sutter said Mantha wants to learn and feels he is already "a better player now than when he joined the team 11 days ago. That comes from our conversations. He understands that he's not going to get on the ice if he's only going to be a power-play guy."
He got a passing grade in two-way play against the Germans, who were overmatched but took advantage of some early Canadian sloppiness to keep the score close through the first 15 minutes of play.
Mantha has a remarkably quick release for a big man, and having the clever Drouin laying passes on his stick could be a vital asset as the tournament moves on and the competition gets tougher.
He said his grandfather, who scored 109 goals in 556 NHL games from 1957 to 1968 for Montreal, Boston, Detroit and the Minnesota North Stars, was a key influence in his hockey development. He said Pronovost taught him "three key points" to success, but won't reveal them all.
"One is work," he offered.
But he still went into the team's selection camp two weeks ago in Toronto looking to prove he can do more than score.
"For sure I had to make an impression," he said. "Not every player was guaranteed a spot and I had to battle through it. I got great news and now I'm trying to carry it on."
Drouin has played with Mantha before in an exhibition series and feels that chemistry is developing.
"Anthony's a big guy," said the third overall pick of the Tampa Bay Lightning. "He's hard to play against in the corners and everything. I try to use my hands and speed and I think we complement each other pretty well."
The Canadian team got a rousing welcome from the crowd of 1,831 — nearly all wearing Hockey Canada jerseys — at the 5,800-seat Isstadion, but still looked nervous as the game began.
The Germans got the opening goal off a faceoff in the Canada zone as Dorian Saeftel put a shot through traffic that fooled goalie Jake Paterson only 1:35 into the game.
After Josh Anderson and Mantha gave Canada a lead, Janik Moser tied it for Germany after defenceman Adam Pelech whiffed on a clearing attempt.
Then Canada took over as Mantha and Bo Horvat got goals before the intermission, Sam Reinhart and Mantha added goals in the second period and Nic Petan finished a play engineered by 16-year-old Conner McDavid in the third.
McDavid had two assists, while his linemate Reinhart also had two points. Their line with Horvat could have had several if not for some fine saves by Shawinigan Cataractes goalie Marvin Cupper.
"I think I played pretty well," said McDavid. "I missed a whole bunch of chances that I'm pretty embarrassed about, but I thought I played an all right game.
"That shift where Bo and Reino set me up for a couple of breakaways and the goalie made a couple of good saves — I was beating myself up pretty good about it, but they laid off me. They were good about it."
The shots were 30-24 for Canada, but the game was not nearly that close.
A tougher test comes Saturday when Canada plays its second group stage game against the Czech Republic.
Notes: Defenceman Griffin Reinhart sat out the first of three games he will miss due to a suspension incurred at last year's world juniors. . . Germany has six players from Canadian junior teams, including Prince Albert star Leon Draisaitl who had a mostly quiet game, and another six who play in the U.S. . . Canada has its second youngest team ever at 18 years 10 months 25 days. Only the 1987 team that was disqualified after a brawl with the Soviets was younger.