“He was ready to make the jump, maturity-wise and skill-wise,” said the junior forward Jordan Greenway, one of McAvoy’s four roommates last year.
McAvoy had played only four A.H.L. games when the Bruins lost two top defensemen heading into the Stanley Cup playoffs against the Ottawa Senators. They summoned him to Boston, but before he could fly with the team to Canada, he had to stop at his B.U. dorm to pick up his passport and a couple of suits.
He made his N.H.L. debut in April, paired with the Bruins’ captain, Zdeno Chara, who played his first N.H.L. game a month before McAvoy was born in 1997.
The Bruins lost the series in six games, four of which required overtime, but McAvoy picked up three assists and averaged 26 minutes 12 seconds of ice time, second only to Chara.
“Nothing rattles his cage,” said the junior forward Ryan Cloonan, who also lived with McAvoy last year. “Even off the ice, he’s such a mature kid for how young he is.”
But McAvoy’s season still was not over. After joining Bruins teammates for a weekend in Arizona, McAvoy joined the American team for the world championships in France and Germany in May. The United States lost to Finland in the quarterfinals on May 18, ending what McAvoy’s father called “a wild ride.”
Charlie McAvoy Sr., with his brother Kevin, owns McAvoy Plumbing in Long Beach, N.Y., a business started by their grandfather in 1926. That his only son, the second of his four children, was able to handle so much in such a short time, at such a young age, did not surprise him.
“He’s always been a pretty grounded kid, and he doesn’t have my temper,” McAvoy Sr. said. “He stays in the moment. In fact, he’s always telling me, ‘Dad, don’t get too far ahead of yourself.’”
Now 6-foot-1 and 211 pounds, the younger McAvoy started skating when he was 3. After playing for Long Beach High School as a freshman, McAvoy lived with a billet family in Michigan for two years while playing for the United States National Team Development Program.
With the help of his mother, Jen, a former teacher, he took online classes so he could enroll at B.U. a year early. As a 17-year-old freshman, he often competed against players five and six years older, but he led the Terriers’ defense with 25 points in 37 games, earning Hockey East All-Rookie Team honors.
B.U. Coach David Quinn said, “Charlie’s an easy guy to love.”
Quinn added: “I think he’s got an incredible ability, which I think great players have, to have swagger without arrogance. He’s very confident in who he is, and he knows how good he is, but he doesn’t have an arrogance about him that affects his play or his teammates.”
When McAvoy was drafted by the Bruins, the clock began ticking on his college career. Quinn said the only time he sensed McAvoy being distracted was the approach of last season’s N.H.L. trade deadline on March 1, when rumors swirled that McAvoy might be included in a trade to the Colorado Avalanche.
The speculation reached a pitch when Colorado General Manager Joe Sakic took in the Beanpot finals on Feb. 13 between B.U. and Harvard.
The Terriers lost, 6-3, and Quinn said, “Charlie probably didn’t have one of his best games.” Shortly thereafter, Bruins General Manager Don Sweeney told McAvoy he would not be going anywhere.
“I thought Charlie played his best hockey after the trade deadline,” Quinn said. “He was able to play hockey with no distractions.”
For McAvoy’s N.H.L. debut, the Bruins flew his parents to the game in Ottawa. When McAvoy saw his father after the game, he smiled and said, “Dad, I just played in the N.H.L.”
For the rest of the playoffs, McAvoy lived in a hotel in Watertown, near the Bruins’ practice facility. During the summer, he moved into an apartment in Boston with Matt Grzelcyk, a former college teammate who is also with the Bruins. He worked out at B.U.’s Agganis Arena.
He made it home to Long Beach a couple of times, including a few days in July when his parents threw a small party for him to celebrate his year.
His mother noted McAvoy’s embrace of his new responsibilities as a professional, even in what foods he would eat.
“Everything he has had to do has taught him something new to get to the next step in his life,” she said. “He keeps making more mature decisions along the way. I think he’s grown a lot over the last four months.”
What the Bruins like about McAvoy, who turns 20 in December, is his ability to move the puck up ice, a good fit for Coach Bruce Cassidy’s aggressive style. And he is not shy about using his size in the defensive end.
Cassidy also has been impressed by McAvoy’s makeup.
“Things don’t bother him,” he said. “And he always seems to play better on the bigger stages.”
An earlier version of this article misstated the given name of Charlie McAvoy Sr.’s brother and business partner. He is Kevin, not Peter. It also misstated the surname of a Boston University hockey player. He is Ryan Cloonan, not Clooney.