Fifteen seconds is all it took to understand why Giannis Antetokounmpo is unlike any NBA player who came before him.

Here’s what the Milwaukee Bucks’ star did in those 15 seconds at the end of the fourth quarter on Saturday night. He poked the ball away from a shooting guard. He caught an outlet pass as he crossed halfcourt and required exactly one dribble before he dunked as three defenders tried to catch him. And then he stuffed a 7-footer who had the bold idea to believe he could dunk over him. There may not be a better sequence of plays this entire season.

There are lots of crazy things about this player who is now known more for being a freak than Greek. Antetokounmpo is 6-foot-11 and often looks about six feet taller. He’s averaging 36.8 points and still hasn’t shot well. He is somehow only 22 years old.

But the single craziest thing about this player who can harass a guard on the perimeter, cover half the court with one dribble and then reject a center at the rim is his position: Antetokounmpo is often used as Milwaukee’s point guard.

“Crazy has started to become normal for him,” Bucks center Greg Monroe said.

There is no position in all of sports that has changed more in such a short amount of time as the big man in basketball. Bigs are the most interesting young players in the NBA. And what makes them interesting is they no longer play big.

“It was always the running joke when I was growing up: Point guards want to be big guys, and big guys want to be point guards,” Memphis Grizzlies coach David Fizdale said. “Now we’re actually seeing that happen.”

Being tall isn’t enough to play in today’s NBA. The game’s tallest players now must be able to go small, too. This is the most profound shift in basketball since the introduction of the 3-point line, and people around the league say they’re not sure anything can stop its momentum.

“Maybe if Wilt Chamberlain comes back?” Rockets coach Mike D’Antoni said.

There are many reasons the game isn’t going back to the way it was. NBA teams have enough data to understand that centers backing down their defenders is less efficient than other plays even for the biggest of big men, which is why post-ups were down 21% last season from 2013, according to Stats LLC.

Philadelphia 76ers guard Ben Simmons passes the ball over Toronto Raptors guard Fred VanVleet during a recent game. Photo: Frank Gunn/Associated Press

That’s not the only secret that’s out. NBA teams also have realized something that they failed to comprehend for many years: 3-pointers are worth more than 2-pointers. “Mathematics doesn’t change,” D’Antoni said.

Then there is the shadow of the Golden State Warriors. The seemingly unstoppable team’s most unstoppable lineup has Stephen Curry, Klay Thompson, Andre Iguodala, Kevin Durant and Draymond Green. It does not have a traditional center. And its competition has downsized to keep up.

Anyone who’s been around the NBA long enough understands the game is a marketplace of ideas. One team finds an inefficiency and exploits it until the league corrects and the inefficiency suddenly lies elsewhere.

But that may not happen this time. NBA teams have decided they can’t afford to play an immobile big man because the league’s best teams will isolate and roast him on the perimeter every possession. That’s why the modern big man is someone who can do everything: play in the post and the perimeter on offense, protect the rim and the 3-point line on defense.

“You’re watching the big men evolve,” Fizdale said. “I see us in the middle of the evolution, and I don’t see when it’s going to switch back to the old days.”

That evolution of bigs is the driving force behind the game tilting in favor of the offense. It’s simply easier to score with more space in the halfcourt and less crowding around the basket. Which explains why NBA offense peaked last season.

It already feels obsolete to call these players unicorns because they’re an increasingly common species. They have combined their size with skill in a way that makes NBA executives wonder if it’s possible to play adequate defense against them.

There are almost too many to fit in one paragraph, but here goes anyway. The Philadelphia 76ers already had Joel Embiid, who has the potential to be so good they gave him a contract extension after playing 31 games in three years, and now they have 6-foot-10 rookie point guard Ben Simmons, who recorded a triple-double on Monday night. The Minnesota Timberwolves have a foundational piece in generational talent Karl-Anthony Towns. The Denver Nuggets have a center in Nikola Jokic who is already the NBA’s best passing frontcourt player. The New York Knicks are re-building from their latest re-build around Kristaps Porzingis. And the New Orleans Pelicans are running pick-and-rolls with Anthony Davis and DeMarcus Cousins.

Some bigs have become forwards. Some bigs have been recast as wing players. But the most intriguing big is Antetokounmpo because he plays like a point guard.

“Sometimes I’m going to be the one scoring the ball. Sometimes I’m not,” he said last week. “I can find my teammates open shots. Or I can make open shots.”

It isn’t too soon to say that Antetokounmpo could be the league’s most valuable player this season. He’s averaging 36.8 points, 10.8 rebounds and 5.3 assists through four games. What makes that even scarier for other teams is that Antetokounmpo has only made one 3-pointer despite closing every practice shooting threes with Thon Maker—who is 7-foot-1 himself. At that one basket is the entire team’s identity.

“We have a bunch of long, athletic, dynamic players who are multi-positional,” said Milwaukee general manager Jon Horst.

Bucks coach Jason Kidd admits that Antetokounmpo would’ve been a center when Kidd was a point guard for no reason other than Antetokounmpo happens to be taller than most people on earth. “Bigs were to stay in the paint,” Kidd said. “But everything has changed. They’re on the perimeter. They shoot the three. They move like guards.”

That’s because they are guards. Antetokounmpo controls the ball. He initiates the Bucks’ offense. He uses his size to survey the court and pass over defenses. “When you have that advantage,” Kidd said, “it makes the game a little different.”

It’s the people who know the NBA best who say they have never seen anyone like Antetokounmpo. The same was said years ago, in fact, about one person who recently declared that Antetokounmpo could become “the best player to ever play.”

That player without precedent who anointed this player without precedent was Kevin Durant.

Write to Ben Cohen at ben.cohen@wsj.com