BOSTON — The chants rained down from the TD Garden rafters late in the Boston Celtics’ Game 1 win over the Philadelphia 76ers on Monday night.

“He’s a rookie! He’s a rookie!”

Jayson Tatum is a rookie. A freakishly good rookie. And he was supposed to be “the other rookie” in the Celtics’ second-round NBA playoff series, with presumptive Rookie of the Year Ben Simmons running the show for the 76ers.

Not anymore.

Tatum was the best player on the floor for either team in Boston’s dominating 117-101 victory, scoring a playoff career-high 28 points on 8-for-16 shooting, while going 11-for-12 from the charity stripe. The Duke product, who just as easily could have suited up for the 76ers had Danny Ainge not traded the No. 1 overall pick in the 2017 NBA Draft to Philly for the No. 3 selection, was aggressive from the opening tip, scoring at all three levels and out-dueling his rookie counterpart in Simmons, who scored 18 points but struggled to make a significant impact.

“Just to be aggressive,” Tatum said about his mindset before the game. “Especially with (Jaylen Brown) out, that’s 20 points mostly on the bench, so I tried to make up for that.”

Not only was Tatum deadly on the offensive end, but he also was locked in on the defensive side of the ball. The rookie forward shifted down a position with Jaylen Brown out, meaning he was given the task of chasing 76ers sharpshooter J.J. Redick and swingman Marco Belinelli around the court. Tatum’s tight closeouts and willingness to fight through screens were part of the reason Boston held the 76ers to 19.2 percent from beyond the 3-point line.

The 20-year-old did almost everything well Monday night.

The Sixers opened with the smaller Redick guarding Tatum, and he immediately took advantage with his size by getting to the free-throw line five times in the opening period.

Tatum also provided a counter punch for Boston when Philadelphia appeared to be making run in the second half. With the 76ers trailing by eight midway through the third period, Simmons drove the lane and missed a floater that Al Horford rebounded. Tatum immediately took off down the court, received the outlet pass and threw down a monster dunk to bump the lead back to double digits and blow the roof off TD Garden. The Celtics swelled the lead to 17 after Tatum’s fast break bucket.

All in all, Tatum outplayed his rookie counterpart and could-have-been teammate in Game 1, but he claims he isn’t focused on individual matchups.

“I think at this point I’m just focused on winning,” Tatum said after the game. “It’s not really about matchups, I just want to win and advance. It’s a team thing right now.”

When asked if he thought he was a better rookie than Simmons, though, Tatum offered a big smile and chuckle.

“No comment,” he said.

Thumbnail photo via Winslow Townson/USA TODAY Sports Images