BOSTON — Joe Mazzulla, the stoic 34-year-old first-year head coach of the NBA championship favorite Celtics, did not crack a smile on the podium in the afterglow of his first career win, but he did crack a joke.

With four minutes remaining on opening night and his team firmly holding a 13-point lead against the Philadelphia 76ers, Mazzulla issued his first coach's challenge. As officials reviewed a foul call against Celtics veteran Al Horford that they would eventually overturn, Mazzulla gathered his team in a huddle.

"I said, 'The most important thing is, don't listen to anything I'm saying,'" he deadpanned.

It was the closest he came to showing any emotion in front of reporters. It was also indicative of the two tenets he has preached to his charges in the month since news broke that he would serve as interim coach during the one-year suspension of Ime Udoka, who coached the Celtics to the NBA Finals last season: 1) The importance of poise, which escaped them in their title series loss to the battle-tested Golden State Warriors, and 2) the sense that navigating this season is a partnership with his players, not a dictatorship.

"The thing I like about Joe and admire about him is that he's very honest that he doesn't know everything," said rising Celtics superstar Jayson Tatum, who was the best player on the court in Tuesday's 126-117 statement win. "He wants us to help him out as much as he's helping us out. It's like we're in a relationship. We're all on the same page and trying to accomplish the same thing, so we're all in this together."

This Celtics season could go two ways. They could fracture in the aftermath of a crushing Finals defeat, followed by the sudden loss of the head coach who forged their identity. Or they could rally around Udoka's replacement, and early signs are pointing down the latter path, at least until any eventual adversity hits.

"I'm grateful for the relationships that we have," Mazzulla added. "We talked before. These guys have been through a lot together, and they're great players, so it's just a matter of working together. I appreciate their trust — their buy-in — but they come up with a lot of good stuff as well, and we kind of just figure it out."

This is no lip service. Mazzulla has matured into a relationship builder. You could sense it in the postgame moments he spent outside the press room, where a horde of his family and friends awaited his embrace.

"Thank you," he told one member of his staff who congratulated him on his first win. "I appreciate you."

Behind closed doors, Mazzulla marched into the locker room after his head-coaching debut and held a captive audience, preaching togetherness. "Nothing gets in the way of winning," he said, before Marcus Smart broke the serious tone. "Especially that first one," said Smart, who led his teammates in dousing their rookie head coach in bottled water. A staffer gifted Mazzulla the game ball, and he cracked a smile.

"He's got a goofy side," the Celtics' Grant Williams told Yahoo Sports. "It takes a little winding to get it out of him, but it was fun to see him smile. Afterwards, he was like, 'Man, f*** you guys,' but at the end of the day he's happy, he's smiling. I know when he goes home he's gonna be screaming, hyped, doing everything he can't do here, because he has to stay composed. He's gonna be a phenomenal head coach."

It is Mazzulla's poise that the Celtics reflected in the third quarter of their opener. The game's first 26 minutes saw eight lead changes, but Boston guard Derrick White's floater two minutes into the second half marked the last one. Over the next eight minutes, the Celtics methodically built a double-digit lead — one they surely would have blown to start last season, when they took a sub-.500 record into late January.

This time around, they never relented. The Celtics' smothering defense forced five Philadelphia turnovers in the third quarter, leading to nine of their 24 fast-break points. In the halfcourt, Tatum roasted all comers, scoring 17 of his 35 points in the third. Mazzulla felt confident enough to field a reserve lineup of Malcolm Brogdon, White, Sam Hauser, Blake Griffin and Williams to start the fourth quarter. They held the fort until Jaylen Brown scored 13 of his 35 points over the final 10 minutes. It was the Sixers — a popular pick to take the East crown from the Celtics — who malfunctioned on offense and borderline quit on defense.

Even Smart, the beating heart of Boston, found his version of poise amid freneticism. He said Sixers star Joel Embiid "tried to break" his arm during a skirmish 19 seconds into the second half. Smart grabbed the right leg of Embiid, who took a tumble, and Brown stepped between them before cooler heads prevailed.

"It's maturity," said Smart. "I could've cracked his head open, but I didn't, and that's the maturity we have."

Mazzulla, universally lauded by his players for his scouting ability, never flinched. He preached execution throughout his team's early foul trouble. When spacing on offense failed the Celtics early, he urged them to fight for it, and they did. He vacillated between small ball and double bigs. They had a clear game plan to bring help when Embiid launched into his post moves and succeeded in stifling a handful of possessions.

"From the start of the game, you could see his confidence just oozing out," said Smart. "Most coaches, they're trying to figure things out. He comes right in like he's been doing this for years, and that's the beauty of having someone who's been with the team as long as Joe has. We have the utmost confidence in him, and he has the utmost confidence in us, and we just have to go out there and do what we have to do."

Likewise, Williams knew Mazzulla had this head-coaching thing figured out from their first practice. Not this season, but in 2019, when Mazzulla first joined Brad Stevens' staff as an assistant for Williams' rookie year.

"First day," said Williams. "I've known that since the first day I met Joe. You could just tell the passion and the analogies he was using — his way of wisdom. He just did a phenomenal job in terms of details as a coach. He encourages everyone around him, and that's what you love about a leader. I've been confiding in him since I was a rookie, and I'm just happy to have him really step into that role, which he deserves."

"You could always see that Joe had the makings of being a head coach some day," added Tatum, who was 21 years old when Mazzulla was hired from the Division II college ranks. "He was very motivated, relatively young, so he's got a lot of passion, and he's just overall a good person. As much as I do on the court, he cares about the things I do off the court, and we've had a great relationship ever since he's been here."

Tatum stopped mid-sentence to wipe something from his brow, and he too had jokes.

"I swear I ain't crying," said Tatum. "I've got something in my eye."

The All-NBA first-team forward then continued. "We've had a great relationship ever since he's been here," he said, "so it's only going to continue to grow as we grow in our careers together and hopefully win a lot of games."

The vibes in Boston, they're immaculate, at least for the first of those wins with their freshman head coach.

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Ben Rohrbach is a staff writer for Yahoo Sports. Have a tip? Email him at rohrbach_ben@yahoo.com or follow him on Twitter! Follow @brohrbach