Eli Manning leaves after a loss against the Tennessee Titans...

Eli Manning leaves after a loss against the Tennessee Titans at MetLife Stadium on Dec. 16. Credit: Jim McIsaac

Pat Shurmur has remained true to his word — and to his plan — with Eli Manning.

With his 37-year-old quarterback showing disturbing signs of slippage during the first half of the season — some of it through no fault of his own — Shurmur never deviated from his bottom line: Whichever quarterback gives his team the best chance to win, that’s who his quarterback will be.

In the end, it turned out to be Manning — for the entire season.

And that may very well be the case in 2019, too — albeit with different results, Shurmur hopes.

Manning and the Giants took advantage of a much-needed midseason bye week and returned to show the form that Shurmur and general manager Dave Gettleman had envisioned when they decided to remain committed to him. The Giants won four of their next five games, briefly flirting with an unlikely playoff run, and Manning benefited from improved blocking to more closely resemble the reliable quarterback he had been for most of the previous 14 years.

So Shurmur never got to the point of seriously considering going with either of his two backups — Alex Tanney or rookie Kyle Lauletta. He figured out a way to better incorporate rookie Saquon Barkley in the offense, giving Manning a more reliable play-action passing game and giving the Giants a chance.

In the end, there were two key stumbles along the way — a frustrating road loss to the defending Super Bowl champion Eagles and an absolute no-show in a shutout loss to the visiting Titans. The Giants still have earned only one playoff berth and no playoff wins since Manning won his second Super Bowl MVP after the 2011 season.

In a perfect world, Sunday’s Giants-Cowboys regular-season finale would be a dramatic showdown with a shot at the NFC East title. Instead, the Giants’ disappointing season will end against a team that already has clinched the division title.

Though there still is some uncertainty about what lies ahead, the Giants might not have a better option than sticking with Manning in 2019. Eagles backup Nick Foles might be on the free-agent market. Same with Saints backup Teddy Bridgewater. But Manning might have bought another year with his improved play in recent weeks.

There will be a strong cast of offensive players returning in 2019 — including Barkley (who might win Rookie of the Year honors), receivers Odell Beckham Jr. and Sterling Shepard, and tight end Evan Engram. The Giants hope to bring back right guard Jamon Brown, who solidified the offensive line at midseason. Rookie Will Hernandez will return as a much more experienced player in Year 2.

The question is: How much better can the Giants be with Manning in 2019?

The answer depends on Manning himself and whether he can play at a high enough level, and also on what happens around him. And not just on offense. Gettleman needs to upgrade his defense, especially after watching the Giants blow several second-half leads, including last week in a 28-27 loss to the Colts. He’ll have to replace Olivier Vernon, who once was looked to as the centerpiece of defensive coordinator James Bettcher’s group but looked disinterested for much of the year after returning from an ankle injury. The run defense suffered after they traded Damon Harrison to the Lions, so beefing up the interior of the line is another priority. So is cornerback depth and, depending on whether Landon Collins is re-signed, safety depth.

Oh, you can add “linebacker upgrade,’’ too.

And what about quarterback?

It’s time to give serious consideration to drafting Manning’s heir apparent — just as it was last year, when Gettleman instead chose to take Barkley, the player he considered the best talent in the draft. He passed over USC’s Sam Darnold, a decision that certainly benefited the Jets but might be one that the Giants regret in the years ahead.

With Darnold looking better as the season progressed and with Manning nearing the end of his career, the Giants will need to find a replacement ASAP. Justin Herbert’s decision to return to Oregon in 2019 doesn’t help, and next year’s class isn’t considered as deep as 2018, when four quarterbacks went in the first round. All four are now starting, leaving the Giants to wonder what might have been.

Barkley is a terrific player, no doubt, but kicking the can down the road at the most important position on the team may burn them in the end. That said, Gettleman has been closely scouting next year’s draft-eligible quarterbacks, so maybe there is an answer there. Maybe Dwayne Haskins of Ohio State or Drew Lock of Missouri or Ryan Finley of North Carolina State can become their next starter.

Or maybe Gettleman will have to wait until 2020, when a better class of passers could be available.

For now, their most expedient option appears to be Manning, who hasn’t missed a start because of injury since taking over as the No. 1 quarterback in 2004. The only time he missed a game occurred when Ben McAdoo mishandled his quarterback situation late last year and paid the price with his job.

Manning told me this week that he believes he still can play at a high level. He said he’ll know when it’s time to walk away.

“I think you know when it’s done or when your body can’t do it, or you can’t stay healthy or you can’t make the throws,” he said. “So I still feel strong. I think I can still play and still make the throws and run an offense and win football games.”

He has a chance to win one more Sunday against the Cowboys. There’s a chance this could be his final game in a Giants uniform, but the greatest quarterback in club history seems ready to give it another shot in 2019.

And the Giants seem ready to give it to him.

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