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

Dave Gettleman doesn’t only need to be right about Daniel Jones.

He needs to be right about Pat Shurmur.

Because Jones’ development will be in Shurmur’s hands now.

The passing of the torch from Eli Manning — when and why — will be determined by Shurmur.

There were two overriding reasons why Gettleman hired Shurmur and ownership signed off on him:

He was an adult.

He was a quarterback whisperer and superior offensive play-caller.

Shurmur was deemed to be the right man at the right time to squeeze whatever was left out of Manning’s right arm and simultaneously groom his successor one day.

We now know that one day will arrive, and it will arrive sooner rather than later.

And whenever it happens — sometime this season or at the beginning of the 2020 season — Shurmur will have to be certain that the kid will not be a Big Blue deer in the headlights.

Shurmur will survive another season without a playoff berth in 2019 if for no other reason than the last thing ownership would want is another head coach who lasted only two seasons a la Ben McAdoo (and Ray Handley).

After all the rebuilding, the trade of Odell Beckham Jr. and the departure of Landon Collins, this isn’t about 2019, no matter what kind of spin you will be hearing out of East Rutherford.

This is about 2020.

For the first time in his HOF career, Manning knows the organization has bigger plans for his backup quarterback than to hold a clipboard on the sidelines and hope and pray he never plays once the preseason ends.

Manning won’t have a No. 1 receiver and he won’t have a defense that anyone fears, except for Giants fans.

In the likelihood that another winter of discontent were to loom ominously towards the second half of the season, many Giants fans will be torn between itching to see the future and dreading a sad ending to a champion and a championship past.

It will be Shurmur — with input from Gettleman and ownership, naturally — who will have to make the delicate, franchise-altering decision as to when it will be the right time to take the ball from Manning.

It was botched two years ago, you might recall.

Back in 2004, Manning played seven games as a rookie after replacing Kurt Warner — who was 5-4 — and in his second season got Tom Coughlin to the playoffs.

Shurmur may be faced with a similar scenario.

Manning will now be under the gun to win, and win quickly.

“I think we’re going to play the very best player,’’ Shurmur said last month. “I know we’re dancing around the words there. But right now Eli’s getting ready to have a great year, Daniel’s getting ready to play. And you just see what happens with it.’’

He also said: “We feel good where Eli is, he’s our starting quarterback, and we’ve got a young player that we think is going to be an outstanding player getting himself ready to play.’’

Shurmurologists will be attempting all summer and autumn to figure out exactly what he meant.

If Shurmur misses the playoffs for his fourth straight time as a head coach — two with the Browns — he will be under the gun more than he ever has been in 2020 to make the playoffs with Jones.

Shurmur will be walking a tightrope between hoping Manning will be better with a second year in the offense and getting Jones’ feet wet enough to have a chance to hit the ground running in 2020.

Gettleman and the Giants will need Shurmur to be the kind of quarterback whisperer for Jones that he was for Case Keenum with the Vikings.

Manning will be the good soldier and mensch that he is. But he undoubtedly recognizes that you sit the sixth pick of the draft for only so long. Daniel Jones better be ready when he gets the keys to the kingdom. It will be up to Shurmur that he is.

That there will be Life After Eli Manning.