The biggest indicator that Jayson Tatum is a franchise player, one of the most prolific scorers in the NBA, a generational talent, is days like Sunday against the Nets are no longer a surprise, or an eye-raiser.

Tatum’s 54-point masterpiece as the Celtics edged the rival Brooklyn Nets, 126-120, in front of a packed, mostly unmasked house at TD Garden was just another example of how much the 24-year-old has blossomed into an offensive juggernaut.

He bested mentor Kevin Durant, who had 37 and looked unstoppable most of the afternoon, and Tatum took over the game when the Celtics were clinging to hope down 9 points late in the third quarter.

In November the Celtics would have let go of the rope, succumbed to Brooklyn’s star power, played like the inferior team. But behind Tatum, who scored 18 points in the pivotal final quarter, the combined total of Durant and Kyrie Irving, the Celtics bested another game opponent, and Tatum’s legend and reputation grew even larger.

He turned 24 earlier this week and is already 20th on the Celtics’ all-time scoring list. And what has led to this ascension is a soaring confidence, regardless of who is on the other side, Durant, Irving, LeBron, Giannis, Curry, he is the baddest man on the floor.

He has to carry that mind-set. That’s the only way you become one of the greats, by truly believing you are and exceeding the level of expectation. Tatum is 10 years Durant’s junior, he has fresher legs, he’s stronger, and more athletic. So he has to feel like he’s better on these nights, even though he may not be on the other 361 nights.

“As a competitor, especially games like this, those matchups, playing one of the better teams, with two of the best guys,’’ he said, “these are the moments as a kid you dreamed about and you looked forward to. So when those opportunities come, you try to make the most of it.’’

Tatum was 15-for-30 shooting and made 8 of 15 3-point attempts, which has been his troubled shot this season. But when it counted, when the Celtics needed a bucket, Tatum bypassed settling for 3-pointers and attacked the basket with his grace and ability to score around multiple defenders.

He scored 10 consecutive points in the fourth, staving off a Brooklyn rally when the Nets took a 1-point lead with a 9-0 run. Tatum was not about to let the Celtics lose on this stage, in front of a nationally televised audience, in front of Glen “Big Baby’’ Davis, who after being booted from a front row seat, planted himself in his proper second-row seat and trash-talked Durant.

“It’s impressive,’’ Celtics coach Ime Udoka said of Tatum. “At times you forget they all are out there. Taking that alpha approach to being the best guy on the court each night and that’s what’s needed against some of these high-level opponents. He takes pride in the matchups going against high-level guys and we’re not making it a one-on-one thing but he wants to play well and takes pride in it.

“Impressive for sure, but not unexpected.’’

Tatum’s been good enough this season to earn his third All-Star appearance and become a top 10 scorer but he had admittedly struggled with his playmaking and 3-point shooting. Tatum wasn’t his efficient self in the early season but he has become more of a facilitator, more of an attacker, trusting his natural abilities more.

In other words, he has become more of a superstar with more non-superstar behavior: making the right pass, passing up a good shot for a great shot, driving to the rim to draw fouls and give himself and the team easier points.

“He’s playing the right way,’’ guard Marcus Smart said. “He’s picking his spots. For us and for Jayson, his growth is very, very important. He’s been showing not just himself but us and everybody in the world that he knows what he wants to work on and he’s doing it. He’s playing the game a lot smarter and it’s showing.’’

It’s not that Tatum wasn’t playing intelligently before, but he played with a lack of trust for his teammates. He felt as if he had to do it all. He watched Kobe, Durant and other mentors score at will, carry their teams to victory and Tatum wanted to be the same way. But he is learning how to more consistently and more efficiently.

“When he’s having one of his big offensive games like that, it loosens up everybody on the court,’’ Irving said. “They were just punishing us down the stretch. JT was getting to his spot. He was definitely on his game. You got to give respect to him. He got to the free throw line. He just got in a good rhythm. I’ve seen it. I was his teammate.’’

Tatum wasn’t really emotional about this performance. He’s done this before. He’s scored 50-plus points five times now, each one of them brilliant in its own way but more predictable as his game and his confidence blossom fully.

“When you kind of get in that zone, the basket just seems a little bit bigger,’’ he said. “And you feel just a little bit better about yourself getting to your spots. Ultimately, just trying to make the right play, attack the mismatch and they over-help and find the open guy. That’s it.’’

Sounds simple. But it’s a methodical transformation that has turned Tatum into an elite player.

Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him on Twitter @GwashburnGlobe.