Eli Manning has mostly defied time during his 14-plus seasons as the Giants’ quarterback, having never missed a game because of injury. At 37, as one of the most durable players in NFL history, he doesn’t even look much different from when he took over as the starter midway through the 2004 season.

But his play tells a different story, and what we’re seeing from him this season might not bode well for his future as the team’s starting quarterback.

“He looks old out there,” NFL Network and Fox Sports analyst Brian Baldinger told Newsday. “His reactions look old. He’s never been a guy that can just put the ball in the [right] spot consistently. He’s had stretches of that, but he looks old.”

At 1-3 and with the Giants’ offense showing disturbing similarities to the previous two years, during which the team failed to score as many as 30 points in a game, the focus continues to be on Manning and whether he still can be the same quarterback who was good enough to win two Super Bowl MVP awards.

He has completed 74.2 percent of his passes — far surpassing his career average of 60.1 percent — but has thrown only four touchdown passes. A quarterback once known for his ability to throw authoritatively down the field, he has been content to check the ball down on shorter routes. Odell Beckham Jr., who is off to a quiet start and still hasn’t scored a touchdown, has even questioned why Manning isn’t throwing deeper passes.

The muted numbers may be due to a combination of the conservative play-calling of coach Pat Shurmur and leaky offensive line play, but it’s also a reflection of the fact that Manning simply may not be equipped to overcome the obstacles he once surmounted on his way to a potential Hall of Fame career.

“You have to be aggressive,” said Baldinger, who has closely studied the Giants’ offense on video. He questioned several plays from last Sunday’s 33-18 home loss to the Saints.

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“Checking down on third-and-14 [to receiver Sterling Shepard] against a zone defense, you might as well just punt. If it’s third-and-8, why are you going to Russell Shepard when he’s covered by [cornerback] Marshon Lattimore? You’re not going to win that route. The ball was 5 yards out of bounds. He didn’t have a chance.”

Manning’s tepid performance against the Saints (he threw 41 times for only 255 yards and one touchdown) came a week after he looked sharp against the Texans in the Giants’ only win of the season.

Shouldn’t that performance in Houston be a sign that Manning still has it, that he still can summon the terrific play that made him one of the NFL’s top quarterbacks during his prime?

Not necessarily.

“He was really good against Houston, but that’s what happens when you get old,” Baldinger said. “You have to string these weeks together. You have to start putting some good games together. That was a horrendous [Saints] defense, and the Saints are probably thrilled to get out of there, considering the way they looked the first three weeks of the season.”

The Giants banked on Manning returning to form this season, with first-year general manager Dave Gettleman taking a win-now approach in revamping the team’s roster. He signed 30-year-old left tackle Nate Solder to the richest deal for any NFL offensive lineman, ignored all of the quarterback prospects in the draft and took Penn State running back Saquon Barkley at No. 2 overall, and re-signed Beckham to a $95-million deal, making him the league’s highest-paid receiver.

But Beckham already is expressing frustration with the slow start, questioning whether enough of his teammates are playing with heart and wondering why Manning isn’t getting the ball down the field more. And though Shurmur insists his team does play with heart and Beckham is simply anxious to start winning again, the grumbling will only get worse if the losing persists.

For his part, Manning remains predictably unruffled by the criticism about his play and the Giants’ paltry offensive production. He has made a career of not letting the noise affect him, and that’s still the case as questions mount about whether this team can produce a meaningful season after last year’s 3-13 debacle.

“Obviously not where we want to be, but I see a team with a lot of new guys and new things going on. I feel like we’re getting close,” Manning said. “I see a team that prepares very hard, they practice hard, they’re doing all the right things, and we got to keep doing those things. Keep our head up and just find ways to put it all together and find ways to win.”

Manning has been one of the most positive athletes we’ve ever seen, especially in New York, where the pressure can break even the most resilient players. Even in his two championship seasons, there were moments of doubt about his ability to lead his team to victory, yet he has burnished his reputation with a cool demeanor and extraordinary play in clutch situations.

But there has been only one playoff run since the last championship following the 2011 season, and his own production has steadily dwindled. At his current pace, he would finish with only 16 touchdown passes, his lowest total for a full season.

Despite his outward confidence, Manning may be hiding some rare self-doubt. At least that’s the opinion of Hall of Fame quarterback Brett Favre.

“You start questioning, ‘Do I still have it? Do people still believe in me? Do people still believe in me? Is it my fault? What else can I do?’ ” Favre said Wednesday during a SiriusXM NFL Radio interview. “All those thoughts crossed my mind.”

Favre added, “Now, you may not give the impression to anyone else there is this doubt, but I would think that every player who has played a really long time — as long as Eli, maybe longer, maybe not quite as long — questions whether or not they still have it.”

Favre believes things can get better for Manning with better blocking.

“Given good protection, I think you see an Eli that we have known to be there at the end,” he said.

Baldinger isn’t so sure. He wonders whether Manning is now a step behind his Class of 2004 quarterback peers.

“Philip Rivers doesn’t look old,” Baldinger said. “Ben [Roethlisberger] looks old but doesn’t really play old. Eli looks old.”

Time finally may be running out.