LeBron James won’t be happy about this, but he’ll have to step aside this season when the Finals roll around. It’s the Boston Celtics’ turn — ahead of schedule.

The Celtics are on a 13-game winning streak after Tuesday night’s victory in Brooklyn, and the Warriors will be in Boston on Thursday night to get a firsthand look at what Toronto’s Kyle Lowry calls “the best team in the NBA right now.” That might be a stretch, for devotees of the Golden State brand, but the superlatives certainly apply in the Eastern Conference.

And if it really does turn out to be Warriors-Celtics in the Finals, there will be hearty applause from purists everywhere. There couldn’t be a better matchup for showing young basketball fans how the game should be played.

This isn’t a rip on James, for any such notion is folly. He remains the most commanding presence in the league, somehow unstoppable and indestructible as he navigates his 15th season in the league. It’s just that watching LeBron’s Cavaliers is a colossal drag. They will never play consistently good defense (including whenever Isaiah Thomas comes off the injured list), and their ball movement is centered around LeBron — a compelling but ultimately unsatisfying vision.

Things reached the height of absurdity Monday night at Madison Square Garden. For several days, the Cavaliers had been labeled the worst defensive team in the league, with several reserve players calling out the starters for lifeless performance. So what unfolded, on the grand New York stage? The Cavaliers came out embarrassingly flat and trailed 73-50 in the third quarter. “Cavaliers in a stupor!” Walt Frazier proclaimed on the Knicks’ telecast. “Apathetic and pa-thetic.”

The Cavaliers pulled out a victory because LeBron and Kyle Korver (19 points in the fourth quarter) insisted on it, and because the youthful Knicks have no idea how to close out a win. Still, the Cavs were an unsightly mess for anyone who has watched Steve Kerr’s Warriors or Brad Stevens’ Celtics in action.

Match up Golden State with Boston, and you get the two best defensive teams in the league (statistically, in the Celtics’ case; they rank No. 1). The two best at ball movement, with San Antonio not far behind. And the teams best exemplifying the modern-day NBA game, all about lengthy, cat-quick wings who excel on both ends of the floor, passing and cutting and screening their way to open shots while switching, communicating and not giving an inch of ground on defense.

The surprise in Boston, even to the team’s front office, is that it’s all happening so soon. General manager Danny Ainge had an eye on the future when he gave up the No. 1 pick in the draft, trading down to select Jayson Tatum, out of Duke. Ainge seemed unconcerned when he wasn’t able to deal for Indiana’s Paul George. Ainge became intent on trading Thomas, one of the team’s most popular players of recent years, over concerns about Thomas’ health and impending free agency. Two of the teams’ top defensive players, Jae Crowder and Avery Bradley, were lost in trade, and to top it off, prized free-agent acquisition Gordon Hayward suffered a gruesome, season-ending injury in the Celtics’ first game.

All of it suggested a waiting game for the Celtics, who have stockpiled a number of first-round picks, until James either retires or moves to the Western Conference. But none of that seems to matter now. Not with Kyrie Irving in town.

There hasn’t been a more complete point guard in the league at this early stage than Irving, acquired in the Thomas deal and currently wearing a mask to protect a facial fracture (remember the last time Irving wore a mask? Nursing a fractured jaw, in 2012, he had a 41-point game against the Knicks). He made an awfully bold decision, demanding a trade in his desire to escape Cleveland, and had to endure criticism from media members and from former and current NBA players over what appeared to be an entirely selfish act.

“So he wants to be the whole show, away from LeBron? Good luck with that,” went the line of reasoning. In response, Irving told reporters, “I’m not just this one-on-one individual that wants to go isolation every single time down the floor. That’s not how I appreciate the game.”

Behold the revelation: Under the guidance of Stevens, who preaches defense with a religious fervor, Irving has added that long-missing element to his game, while adopting a pass-first mentality and scoring virtually at will — best handle in the league, no questions asked — when the situation demands.

“Sometimes you get labeled things,” teammate Al Horford said, “and it’s frustrating, because it’s like, ‘I’m not what they’re saying I am.’ Kyrie is making effort and hustle plays, he’s been tough on defense, and he’s our leader. I’ve just been blown away. I don’t think people really understand how good he is.”

The Celtics haven’t lost a bit of their tenacity or team harmony, with Irving running the show and two exceptional young players, Tatum and Cal alum Jaylen Brown, quickly blossoming into two-way talents given huge responsibility at the end of games. It’s hard to bench someone who not only has the potential to score 20 points, but attacks the boards and assumes a lockdown mentality on every defensive assignment.

“It sure looks like Boston is the team of the future in the East, with the assets that they still have and their young talent and their coaching, and Kyrie is amazing,” Kerr said after practice Tuesday. “That looks like a team that is going to be at the top of the East for a long time to come. Whether their time is now or the future, that’s to be determined, but they sure look like they want it to be right now. They’re really sound, and they’re motivated.

“Even without Gordon Hayward and that awful injury, Boston is just crushing people. So, it’s going to be really fun to go against them on Thursday. We know how tough it’s going to be.”

All of which makes Boston one of the two most interesting stories in the East — right there with a Philadelphia team led by the singular talents of Joel Embiid and Ben Simmons. The 76ers unveiled a worthy effort before losing in Oakland on Saturday night, making it clear that head coach Brett Brown values nonstop movement and a selfless mentality as much as anyone in the league.

Let’s take it a step further, then: a Celtics-76ers matchup in the Eastern Conference finals. We’ve seen LeBron, and he’s a hell of a show, but a new day is at hand.

Bruce Jenkins is a San Francisco Chronicle columnist. Email: bjenkins@sfchronicle.com Twitter: @Bruce_Jenkins1