MMANN’ UP OR MANN’ OUT

LOSING TO NINERS TONIGHT COULD EXPEDITE ELI’S EXIT

SANTA CLARA, Cali f. — When should it end?

When does an athlete, who’s had a sustained run of success with his team, know when it’s time to walk away?

When does a franchise, for whom a player has been borderline iconic, know it’s time to move on?

These are the uncomfortable questions the Giants and their veteran quarterback of the last 15 years, Eli Manning, are facing right now.

The Giants went all-in on 37- year-old Eli this past offseason and it’ s back fired— in a stunning and dramatic way — as evidenced by their NFC-worst 1-7 record entering Monday night’s game against the 2-7 49ers at Levi’s Stadium.

In an effort to glean some perspective from those who might best know the answers to these questions, The Post caught up with three former quarterbacks from New York’s football past — Phil Simms, Boomer Esiason and Chad Pennington.

All three enjoyed differing levels of success in New York, understand the unique demands and, for all of them, their respective endings were hardly storybook.

Simms, who played all of his 14 NFL seasons with the Giants, was released in 1993 after leading them to an 11-5 record and a playoff victory.

Esiason was released by the Jets in 1995 and played two more relatively productive seasons for the Cardinals and Bengals.

Pennington, despite leading the Jets to the playoffs in every season in which he stayed healthy, was released before the 2008 season after team owner Woody Johnson became smitten with Brett Favre, who was already finished.

“When you’re playing for a long time and your career is coming to an end, it never ends gracefully,’’ Simms said. “And you know what? So what? When it’s over, the players and fans just remember the good.’’

This, of course, is true. Ask Knicks fans how they view Patrick Ewing now despite the fact that he never led the team to an NBA title as promised. As with Ewing and Knicks fans, it’s likely Giants fans will grow to appreciate Manning more after he’s gone because they’ll realize what they’re missing.

But many Giants fans, frustrated and angered by the team’s decision not to draft a quarterback of the future (Sam Darnold) this past spring, are having a diff icult time seeing the forest through those trees at the moment.

Not when the Giants, hav- ing retained Manning, tried to reload the offensive line and drafted running back Saquon Barkley, entered this season with expectations to contend for the NFC East title — not for the No. 1 pick in the 2019 draft.

The mess t he Giant s f i nd t hemselves i n, of cours e, is hardly on Manning, who’s been sacked an absurd 31 times. But the fact is Manning, who counts $23.2 million against the salary cap, has not looked like part of the solution.

Esiason was the most frank and the harshest with his assessment of the Giants-Manning franchise-player situation when he said their relationship “definitely should have ended last year, honestly.’’

“So now they’re stuck here,’’ Esiason sai d. “Maybe t hey’l l have another high draft pick so they’ll hopefully be able to get their quarterback of the future — just like they did in 2004 [when they traded for Manning on draft day]. They just need to redo what they did in 2004, and it’s obvious to everybody.’’

With it having become painfully obvious Manning is playing his last season with the Giants, Pennington felt the most strongly that Manning should go play for another team.

“I still think he has good football to play, but I think it needs to be somewhere else,’’ Pennington said. “I think it would actually be refreshing for him, and it would re-energize him — similar to how it did his brother [Peyton, who f inished his career in Denver and went to two Super Bowls, winning one]. I know that because I experience that myself, and there comes a point when you are no longer a prophet in your own land.’’

Would Manning be willing to play for another team? That’s a question to which only he knows the answer, and he’s not going there right now.

“No, I don’t see that,’’ Esiason said. “I would hate to see that, but it all depends on how he feels. Maybe going somewhere else would refresh him if it was a really good team and he didn’t have to make $25 million to go there.

“I don’t think he’s totally done. Do I think he can still play in the league? Yeah, of course I do. We see there are moments in games where he looks great, but there also is the reality that he’s 37 and he’s eating up about 15 percent of the salary cap.’’

Simms was uncomfortable saying whether or not Manning should go play elsewhere or whether this is the end.

“Let’s let it play out,” he said. “Let’s see what happens.’’

Simms, however, is not blind to the bind his former team finds itself in.

“The Giants a re in a very tough situation,’’ he said. “It’s tough — the record, how they’ve played, where they’re going, the quarterback is older, what do we do? On paper, it was a great plan what they were trying to do.’’

But the Giants are among the worst in the league in scoring and their red-zone TD production has been putrid — the difference in the season, really.

“It ’s tough on the t e a m,’ ’ Simms sai d. “I think they’re caught in a really difficult situation and it’s almost like, ‘ What do we do? How to play it out?’ ’’

With eight remaining games, this is one of the compelling subplots remaining in this lost season: How are they going to orchestrate the goodbye?

“He’s had some u n b e l i evable games — there have been so many off- t he- c har t s hi g h moments that very few people are able to attain, and you can’t lose sight of that,’’ Esiason said. “But then, there’s also a lot of down moments — especially over the last six years [no playoff wins]. I know he’s the beloved son and he’s going to be treated with the respect that way.”

Pennington believes it won’t be long after Manning leaves before Giants fans look past the current and recent disappointments and appreciate what he’s done for the franchise — two Super Bowl wins and MVPs, never missing a game to injury and handling himself with class.

“At the end of the day, we’ll all look back and go, ‘ Wow, what a true pro,’ ’’ Pennington said.

“If you take two Super Bowls in 15 ye a r s , eve r y f ra nchi s e would say, ‘We’ll take that,’ ’’ Simms said.