As training camp approaches next week, Post NFL columnist Steve Serby looks at the biggest characters in the world of the Jets and Giants.

So much changed for Saquon Barkley on March 12, the night Dave Gettleman traded Odell Beckham Jr., the mercurial wide receiver Gettleman didn’t sign to trade.

Gettleman gave up on the tantalizing vision of Beckham and Barkley terrorizing defenses together for years, gave up on Beckham because he was not his idea of a Stepford Giant.

So now he hands the ball and the offense to Barkley, his dream Giant, touched by the hand of God and a gold-jacket guy and all that.

It is Barkley who is the engine of the Big Blue Bus that Eli Manning still drives until Daniel Jones gets his license. And until Jones is ready to drive in the fast lane.

What can the thoroughbred they call Saquad because of his massive legs possibly do for an encore after winning Rookie of the Year honors over Baker Mayfield?

A rushing title.

A feat last accomplished by a Giant in 1951, when Eddie Price rumbled for 971 yards. Tiki Barber’s 1,860 was second to Shaun Alexander’s 1,880 in 2005.

Because as Barkley tweeted several days ago:

“I got a whole nother level that I can tap into ….”

Barkley proved Gettleman right as the best player in the 2018 NFL Draft, but for all his playmaking genius, he could only help the Giants improve from 3-13 to 5-11 … because a running back, even a 1,300-yard, 91-catch weapon, does not impact a team’s fortunes the way the right franchise quarterback does.

Even as Barkley confronts an army of eight-man boxes, his glass is so half-filled he will nevertheless see opportunities to run to daylight behind a bolstered offensive line … even if he would have seen more daylight with Beckham making life easier on him.

Gettleman and Barkley will be forced to continue to answer to the We Shoulda Drafted Sam Darnold mob if the Jets steal the back page of The Post and make a playoff run and Barkley finds himself stuck in the mud of mediocrity, or worse.

Barkley’s charisma, polish, professionalism and otherworldly gifts will make his transition from rookie to vocal leader a seamless one, and he embraces the role. Beckham, by his own admission, was no natural-born leader and struggled trying to be one, as much as the Giants wished he would grow into one.

As popular as Beckham was with teammates, he had a knack for saying or doing the wrong thing, and John Mara reluctantly signed off on an awful trade that cast Beckham in an addition-by-subtraction light. Barkley has a knack for saying and doing the right thing, and is viewed as a distraction only by the opponent.

With a defense in flux and crying out for a pass rusher to protect a suspect secondary, Barkley will be the centerpiece of an offense that will look to control the clock and keep the Big Blue defense off the field as much as possible. In a league where dizzying passing attacks rule the day, the Giants will be relying on a controlled attack that will miss Beckham’s lightning, quick-strike capacity and struggle when they fall behind.

“Building a roster is not just about collecting talent. It’s not just about how fast, strong or talented a player is, but does he fit athletically, intellectually and culturally what you’re trying to accomplish, which is to win a Super Bowl,” Gettleman said.

Barkley fits athletically, intellectually and culturally.

Beckham did not.

Gettleman has told us that you can win while you build. He won’t use the R word, but only a blind man can’t see that this is Rebuilding. He is certain he has fixed the culture and fixed the offensive line. There is plenty more for him to fix.

Barkley is the Giants’ best player, by far. He is the face of the franchise. But it is a franchise in transition, waiting on its rookie franchise quarterback to take the torch from the 38-year-old franchise icon. It certainly hasn’t all been Manning’s fault that the Giants haven’t won a playoff game since Super Bowl XLVI. It was hardly Beckham’s fault that he played in only one playoff game — where, yes, he played poorly and punched a hole in a Lambeau Field wall afterwards — in his first five seasons.

Gettleman believes he now has the right locker-room culture. For now, until Jones develops into the quarterback Gettleman and Pat Shurmur believe he will be, he is counting on Barkley, more than anyone else, to prove that letting Beckham take his talents to Cleveland was not subtraction by subtraction.

Ride the horse.