ORLANDO — Jayson Tatum looked visibly irritated. Borderline furious.

With three minutes left in the second quarter, the Celtics had already given up 60 points to a young Magic team. Paolo Banchero, the top pick in the most recent NBA Draft, had just driven straight past Noah Vonleh for a bucket and a foul. Though Banchero had started inefficiently from the field, he was starting to build more rhythm against Boston’s big men.

Tatum wanted to shut that off. He wanted to wake up his Celtics teammates. And he wanted to keep his team undefeated. So, as Banchero stood on the free throw line, Tatum demanded to guard the young Magic star.

“I ain’t ask for permission,” Tatum said after his team’s 126-120 win.

Tatum didn’t need to ask. When he made his feelings known, the Celtics bent to his wishes. He started the next Orlando possession attached to Banchero on the low block. When Banchero touched the ball one play later, Tatum picked him up at half court and hounded him. After sticking with Banchero through multiple dribble moves, Tatum stripped the ball from Banchero and knocked it out of bounds off Banchero’s leg.

The play seemed to energize the Celtics, who ended the half on a 10-4 spurt before building another run at the start of the third quarter.

“We kind of let them be comfortable,” Tatum said. “Regardless of who it is, any NBA player that’s comfortable and they hit shots, they start playing with confidence, and then you’re in for a ballgame. And that’s what happened tonight. And I felt like they were just too comfortable in the beginning. So I decided to guard him. I take pride in defense, as we all do.”

Even in a road arena, Tatum left the Amway Center court to M-V-P chants after finishing with 40 points on 14-for-21 shooting. As well as he scored the ball, the insistence on defending Banchero did more to show off Tatum’s early-season mindset. On the second leg of a back-to-back, he could have let the Celtics coast but refused to let them settle for a weak effort. In some ways, he wasn’t just setting the tone for Saturday night, but for the entire season. With one demand, Tatum made it clear that every game matters to him. That he won’t stand for any slippage. That the finals loss has shaped his approach to this season and convinced him to take everything seriously.

Tatum went on to score nine points over the first 2:31 of the third quarter, further giving the Celtics control. They weren’t able to pull away from the Magic but felt more pleased about their execution in the second half.

“Just better ball pressure,” said Joe Mazzulla. “Our pickup points. I thought we did a great job in the second half of picking up our pressure and just playing that way consistently.”

Malcolm Brogdon agreed.

“Super important (win),” Brogdon said. “This is a young team we played tonight, but they’re talented. But they don’t have a lot of expectations as far as winning. So it was important that we got this win. This was, I think, a more important win even than the Miami win, just to show that we’re mature, experienced, and ready to take that next step this year.”

Mazzulla actually sounded happy the Celtics experienced a bumpier ride against Orlando. After two charmed wins to open the regular season, against two quality opponents, he wanted the players to understand that some nights will be more challenging. Sometimes another team will play a great game. Sometimes the Celtics just won’t have their best stuff. During those games, they will still need to find their way back to who they want to be. Mazzulla thought the Celtics showed “great leadership” by picking up their physicality after the slow defensive start.

“Just because we had two good games, we can’t create this expectation that things are always going to go our way,” Mazzulla said. “And that we’re always going to feel comfortable. It’s not going to be that way. I thought our guys did a great job of staying even-keeled, poised. They challenged each other. And we know exactly what we need to do. I think part of a great team is building an awareness to when are we playing well and when are we not, and then how do we get back to being ourselves. And I thought we did a great job of that at halftime.”

Mazzulla called Blake Griffin’s second-half dive after a loose ball the highlight of the game. Derrick White scored 27 points, including a rare dunk that surprised some of his teammates. Tatum had help in changing the tide but knew the Celtics needed his energy. For him, the move showed off growing assertiveness. He almost certainly would not have demanded to guard a player early in his career but understands now how much his actions can impact the rest of the Celtics. That’s the power of stardom, for better and worse.

Tatum didn’t always stop Banchero, who is viewed as a budding star. The two know each other pretty well. As a fellow Duke product, Tatum said he has “spoken frequently” to Banchero and has been “extremely happy” to see the 19-year-old rookie’s success. Noting Banchero’s rare combination of height, bulk, and ability to “do everything on the floor,” Tatum said he thinks highly of the youngster not just as a player but as a person. That just didn’t matter to Tatum once the Celtics started wobbling Saturday. He knew he needed to do something to revive his team’s defense. With the Celtics in their worst defensive stretch of the young season, Tatum shifted over to defend Banchero.

“That’s how you show respect,” Tatum said. “I’m not going to let it be easy. I’m going to compete. You’ve gotta go at somebody regardless of if he’s 19 or 30. Nobody took it easy on me my first year.”

Nobody will take it easy on Tatum now. That’s OK. In most cases, he’s the one who sets the terms of the challenge these days.

(Photo of Jayson Tatum battling the Magic’s Paolo Banchero and Wendell Carter Jr. for a loose ball in the second quarter: Nathan Ray Seebeck / USA Today)