LAS VEGAS -- Fresh off winning an NBA title and signing a record contract extension over the weekend, Jayson Tatum arrived in Sin City for his first Team USA training camp practice Monday with a fresh haircut and feeling like a weight was lifted off his shoulders.

"A hundred percent," Tatum said, shortly after participating in his first practice here after being excused from the opening two days of preparation for the Paris Olympic Games. "Just being at the topic of discussion of so many debates or whatever it is ... 'Can he lead a team? Is he a top-five player?'

"There's still a lot of things I guess they can debate. But now I've done something that they can't debate, I've won a championship. Having that under my belt, like obviously it's still conversations to be had or whatever people wanna say, but they always gotta refer to me to as an NBA champion."

There's no arguing that, after Tatum and the Celtics followed up leading the NBA with 64 regular season victories by going 16-3 in the postseason -- second only to the 16-1 mark by the 2017 Golden State Warriors in terms of playoff winning percentage since the league went to four seven-game rounds in the playoffs.

Tatum said the championship parade last month was "the best two hours of my life," and that it was hard to explain how "insane" the atmosphere and energy were in the city that day as Boston reveled in passing its forever rivals, the Los Angeles Lakers, to become the leader in NBA history with 18 championships.

But if anyone was thinking that the Celtics -- specifically, coach Joe Mazzulla -- would be relaxing after winning that first championship, Tatum made it clear that wouldn't be happening.

" Joe is already trying to draw up s--- for next season," Tatum said with a smile. "And I was like, Joe, f--- that. We just won a championship. Enjoy it. Relax.'

"Like, I'm not worried about any sets that he's trying to put in right now. We had a laugh about it, because he's just so driven -- which we all are, but it's like, man, we worked so hard for this. So we got to enjoy it."

Tatum is also enjoying Boston's remarkable level of roster continuity heading into next season. The team has already re-signed centers Luke Kornet, Neemias Queta and Xavier Tillman in free agency and inked both Tatum and All-Defensive team guard Derrick White to long-term contract extensions.

As of now, Boston has retained 13 of its 15 players from last year's NBA championship-winning roster, with Oshae Brissett and Svi Mykhaliiuk being replaced by first-round pick Baylor Scheierman. Nobody is happier about that than Tatum, who said he hoped everyone would be back from last year's team, citing how much team chemistry helped Boston last year.

"We had a hell of a team," Tatum said. "Obviously the guys that did play, but the guys in our 'stay ready' group are extremely talented whenever we called upon them. I think the camaraderie that we had was such a close knit team that we all got along really, really well.

"I didn't really want any changes. I thought we had a really, really good team on all fronts."

One place where Boston will have a change, however, is in its ownership group. Controlling owner Wyc Grousbeck announced a week ago that he and his family would be selling its controlling stake in the franchise. It was a move that came out of the blue to practically everyone -- including Tatum himself, who admitted he didn't know it was coming.

"I was very surprised," Tatum said. "I didn't hear anything about that prior, and we just won a championship. Thought everything was great. But people have decisions they have to make, especially them and their family. As a man, you gotta respect that."

He also said the ownership group had done a good job of reassuring everyone within the franchise that nothing would change moving forward.

What else won't change is Tatum's status as the face of the franchise in Boston, after he agreed to a record 5-year, $314 million extension.

Now, he said, the goal is to follow up that first championship with winning as many as he can in Boston.

"I'm very just thankful and grateful," Tatum said of signing the extension. "It's not about the money. I've been fortunate enough to make enough, more than I ever thought I would ever have and to take care of my family. So for me, just to feel wanted, that they wanted me to be, wanted me for the long haul, for my whole career.

"I've got nothing but love for the city, the fans and everybody in the organization and we just won a championship, and I want try to win as many as I can."