RJ Barrett is 21 years old. Those who root for the Knicks and those of us who comment on them for a living should find a whiteboard somewhere and write that a hundred times.

RJ … BARRETT … IS … 21 … YEARS … OLD.

When Patrick Ewing was 21, he was a junior at Georgetown. When Eli Manning was 21, he was a junior at Ole Miss. When Derek Jeter was 21, he was a minor leaguer at Columbus.

Barrett already has 2 ¹/₂ NBA seasons behind him — abbreviated seasons, but seasons nonetheless. Much like the New York legends who preceded him, the Jeters, Ewings, and Mannings, Barrett was expected to do special things after being drafted early in the first round in 2019. The kind of special things he did Tuesday night in the Garden, where he imposed his will on the Pacers to the tune of 32 points in a 104-94 victory, and repeatedly inspired the crowd to chant his name.

“It was cool,” said Barrett, who called the sound of the fans chanting his name “a lot of fun.”

Yeah, few things in sports are more fun than that.

This was a significant night in the Garden, a home court the Knicks have consistently failed to defend. Julius Randle returned from COVID protocols as, finally, the Julius Randle of 2020-21, playing with ferocity in collecting 30 points and 16 rebounds. Tom Thibodeau called his performance “remarkable” under the circumstances. Randle had to travel in from his home in Dallas after being quarantined in Oklahoma City, and showed up for work unsure if he was even going to play.

And yet the night still belonged to Barrett and, more specifically, to the absolute need for him to develop into a genuine NBA star if the Knicks are to ever develop into genuine NBA contenders. He made his first six shots, including two 3s, and poured in 19 points in a first quarter that the crowd embraced with fourth-quarter enthusiasm. The fans chanted for him again near the end of the third quarter, after he bullied his way to the basket for his 29th and 30th points.

In between, when Randle failed to get off a shot at the end of the first half, Barrett wasn’t bashful about approaching him with a purpose to discuss what went awry on the play. So the young man was showing leadership too.

“We weren’t on the same page,” Barrett would say.

They were for most of the game. When Barrett plays at Randle’s old level, his Tuesday night level, the Knicks are a much more dangerous team.

“When you have guys who are going to command a second defender,” Thibodeau said, “it opens up the game for everybody else.”

This is what the Knicks thought they were drafting out of Duke. Barrett was the third-overall pick, and even though Knicks fans desperately wanted the first pick and Zion Williamson, Zion’s college teammate entered the league with a chance to be a real NBA star.

If the Big Z is never healthy in the Big Easy, well, maybe Knicks fans actually got a favorable bounce for a change in the lottery. But then again, missing out on pick No. 2 might end up being more painful than missing out on pick No. 1. Ja Morant is tearing it up, dunking hard on people and dropping 36 on Brooklyn and averaging 25 a pop. The 22-year-old Memphis point guard is showing the league exactly what a young superstar looks like, night after night after night. And the gap between Morant and Barrett seemed to be growing by the possession.

Though he made a big production jump from Year 1 to Year 2, Barrett has regressed in Year 3. His shooting percentage and 3-point percentage are down, and he’s not scoring or passing like he did last season. Barrett took the Garden court ranked 252nd in the league in player efficiency rating, or 245 spots behind Morant.

This is pretty important stuff, given that Barrett’s ascension is an essential part of Thibodeau’s long-term vision. Randle might someday replicate the powerhouse season he had last year, but before this sudden revival he’d provided strong evidence this year that he won’t top it. And at the height of Randle’s powers, the Knicks weren’t able to get past five games in the playoffs.

Barrett, on the other hand, has time on his side.

“If you look at the great players in the league, there’s a progression to it,” Thibodeau said last spring. “They don’t just start off being great. I’ve had several players over the years that, each year, they’ve gotten better. I think RJ will be one of those guys.”

RJ absolutely needs to be one of those guys. Asked Tuesday night if he felt he could reach the high level he reached against Indiana on a more consistent basis, Barrett said, “I definitely feel like I’m capable.”

Will it happen? Who knows? But this much is clear:

It has to happen.