John Mara told us after his season from hell ended that he expected the Giants to be good again in 2018.

The quickest way to be good would mean drafting running back Saquon Barkley, teaming him with Odell Beckham Jr. and helping Eli Manning feel forever young at age 37.

It would also defy conventional thinking, which states you cannot pass on a potential franchise quarterback when blessed with the second pick of the NFL draft.

Which means this is a huge moment of truth for new GM Dave Gettleman and the franchise.

Barkley is considered by many as the best player in the draft, and among the safest. But he doesn’t solve the Life After Eli problem.

The $150 Million Question — a Kirk Cousins question — is who would?

Giants fans will have to trust Gettleman to answer it. It is easy for so-called draft experts to anoint a Sam Darnold, or a Josh Rosen, or a Baker Mayfield, or a Josh Allen or a Lamar Jackson as The Savior.

Gettleman has to live with the decision. They do not.

If he has the same conviction on one of these quarterbacks that former GM Ernie Accorsi had on Manning in 2004, he should pick him. But he better be right.

The Giants cannot afford for him to be wrong. Ryan Leaf was the second pick of the Peyton Manning draft in 1998. By the Bobby Beathard Chargers.

There is, of course, precedent for passing on a quarterback with the second pick of the draft. Former Giants GM George Young had the second pick of the 1981 draft. The Saints took RB George Rogers with the first-overall pick.

Young selected Lawrence Taylor.

Two years earlier, he drafted Phil Simms in the first round. Simms had thrown for 28 TDs and 33 INTs in his first two seasons. But LT was much more of a sure thing than California QB Rich Campbell, who was drafted by the Packers with the sixth pick.

Gettleman’s task is complicated by the fact that he must also make an evaluation on whether Davis Webb can be the heir apparent without game tape from his rookie season.

How much sense would it make to sign Beckham to a potential $100 million deal if you don’t have the right quarterback throwing to him?

Gettleman and new head coach Pat Shurmur have decided that they can win with Manning in 2018. Gettleman will use much of his modest salary-cap room to find hog mollies for an offensive line that can keep Manning from running for his life and give him a chance.

Barkley would enhance that chance. Should Gettleman have a conviction on a quarterback not named Darnold, Rosen, Mayfield or Allen, he can also decide to trade down if a quarterback-desperate team makes him an offer he can’t refuse, and stockpile picks. And maybe be fortunate enough to still land Barkley … or highly regarded guard Quenton Nelson.

But good luck in that scenario finding a Joe Montana or a Russell Wilson or a Jimmy Garoppolo.

There currently are 15 starting quarterbacks selected with a top-10 pick, 19 selected in the first round, 25 drafted in the first three rounds, 27 in the first four rounds. On the flip side, none of the six quarterbacks not named Eli Manning drafted with the first-overall pick has won a Super Bowl.

There is reason to believe Barkley, behind these incoming hog mollies, could do for Manning what Ezekiel Elliott did as a rookie for Dak Prescott, what Leonard Fournette did as a rookie for Blake Bortles. Each was the fourth pick of their respective drafts. Barkley is a blue-chipper off the field as well.

For what it’s worth, my current take: Love Rosen’s accurate arm, but his durability and personality give me pause.

Love Mayfield’s leadership and moxie, but doubt if he can be Drew Brees or Wilson.

Love Allen’s prototypical size and arm, but his 56 percent completion mark at Wyoming troubles me.

Love Jackson’s Michael Vick-esque skills, but he’s the biggest projection.

This is what I would do:

If the Browns draft Sam Darnold with the first-overall pick, and Darnold is the Big Apple of Gettleman’s eye, a reincarnation of Troy Aikman, he should take Barkley — unless he is seduced by Allen, who would serve as this season’s Webb.

The quarterback wouldn’t help the Giants become good in 2018. The running back possibly could. The right quarterback could help the Giants become great beyond 2018.

Never more critical than now to Gett it right.