EAST RUTHERFORD, N.J. -- Eli Manning walked off the field in the third quarter of Sunday night’s loss to the Dallas Cowboys as television cameras zoomed in on his face. The look was eerily reminiscent to what New York Giants fans had seen countless times before, especially early in his career.

“Manning Face,” as it is known, is a mix of disgust and bewilderment. It was there on more than a few occasions Sunday night and has become more common to see over the past few years, including during Sunday’s 19-3 loss to the rival Cowboys.

Manning had just thrown an off-target pass to wide receiver Roger Lewis on third-and-goal from the 13-yard line that was 7 yards short of the end zone and off target. Lewis made the catch on the ground but had no chance of scoring a touchdown. The Giants settled for a field goal and didn’t score again, with star wide receiver Odell Beckham Jr. out of the lineup. “No part of the offense was functional,” according to coach Ben McAdoo.

Manning finished 29-of-38 for a hollow 220 yards with no touchdowns and one interception.

The quarterback play was far from the only offender, but it wasn’t good. McAdoo angrily barked at a reporter this week who asked how Manning graded out in the contest and failed to provide an answer. Offensive coordinator Mike Sullivan said Manning didn’t play well.

It’s more than Sunday night that has spawned legitimate questions about Manning and the Giants. A game here or there has morphed into a full season and now a trend.

Manning didn’t play particularly well by any standard last season, even if it was somewhat masked by an 11-5 record. The Giants averaged 19.4 points per game last season. The six teams that averaged fewer points had Case Keenum, Ryan Fitzpatrick, Blaine Gabbert, Jay Cutler, Brock Osweiler and Robert Griffin III as their starting quarterbacks at the beginning of the season.

The Giants had Manning, considered head and shoulders above that class of quarterbacks.

But Manning’s 44.6 QBR ranked 27th among quarterbacks last year and tied for the worst of his career. He was also 27th in QBR in 2013, when he threw 27 interceptions. McAdoo arrived the next season with his offense from Green Bay.

QBR is a metric that accounts for his teammates’ contributions -- such as Beckham's taking a 6-yard pass and turning it into a 61-yard touchdown -- and gives credit or a lessened penalty based on the position he was put in by the blocking. It accounts, to some degree, for the Giants’ subpar offensive line that makes life difficult for its immobile quarterback.

Manning is 36 years old and not playing well. It’s possible the inevitable decline is happening before our eyes, only to be masked by his résumé and the line’s poor blocking.

“Combination of age and questionable offensive line isn’t a favorable pairing for Eli,” one NFL executive said prior to the season. “No one seems to think of their [quarterback] situation as a potential weakness. I do.”

Three executives or scouts were surveyed prior to Week 1 about whether they would rather have Manning or Dallas' Dak Prescott as their quarterback for this season. All three chose Prescott, the Cowboys' second-year signal-caller. That might be indicative of where Manning stands at this point of his career.

Cause for concern

The past few years have seen steady decline, even though Manning did compile some impressive touchdown numbers -- including a career-high 35 in 2015 -- in McAdoo’s pass-heavy, quick-hitter offense.

• 2006-12 QBR: 61.0 (Rank: 16)

• 2013-15 QBR: 50.7 (Rank: 25)

• 2016-17 QBR: 44.6 (Rank: 27)

It isn’t just Manning’s QBR that indicates his play has slipped since 2011, when he carried the Giants through the playoffs and into the Super Bowl while facing constant pressure. Manning ranked 19th and 20th in Football Outsiders’ passing DVOA in 2015 and 2016. He ranked 10th and then 22nd in Pro-Football-Reference.com’s adjusted Net Yards Per Attempt (ANY/A) the past two seasons. His 44 interceptions put him third behind San Diego’s Philip Rivers and Jacksonville's Blake Bortles for most interceptions in the past three seasons.

Nothing Manning did Sunday night against the Cowboys assuaged fears that he might be in the midst of his decline. He threw for 33 yards in the first half, and his longest completion, of 12 yards, came on a swing pass in the flat to Sterling Shepard.

Manning didn’t complete a difficult throw -- under duress or not -- throughout the entire half.

“I have to play better,” he conceded. “I think there were some opportunities to make some plays, to get some things going, get into a rhythm, and I can make some better decisions on some things. So, I can do my part to help us out, get things going. You have new players. When the guys get off to a slow start, that’s when the leader has to step up and do their part.”

Manning undoubtedly remains the Giants’ starting quarterback. That’s not going to change no matter how long the offensive struggles persist this season. He’s a four-time Pro Bowl quarterback and a two-time Super Bowl winner.

But he might be as much a problem with the Giants' offense, which hasn’t topped 20 points in seven straight games, as the offensive line. They need their highest-paid player to overcome non-optimal situations, make plays and help their weakest link.

Manning has seven touchdown passes and eight interceptions in his past seven games and might have embarked on a late-career decline, which is no shame.

Legends such as Dan Marino, Joe Montana and Brett Favre all fell off dramatically by the time they were 37 years old. Only a select few all-time greats -- led by Tom Brady, Peyton Manning and Drew Brees -- excelled in the latter stages of their career. They are the exceptions, not the rule.

Eli Manning has said he would like to play into his 40s. He is signed through the 2019 season, but the Giants have also started preparing for the future. General manager Jerry Reese conceded Manning was “probably on the back nine” of his career earlier this year. Reese drafted Davis Webb, now serving as the team’s third-string quarterback, in the third round of the year’s draft.

Geno Smith is the backup. Webb isn’t ready. For better or worse, it’s still Manning’s show.

Reasons for optimism

While there are considerable concerns about whether Manning’s best years are behind him, it would be a mistake to count him out during the final act of his career. There were similar thoughts about his inability to perform when he was in his early 20s and early 30s.

Manning has experienced more than his fair share of ups (two Super Bowl MVP runs) and downs (25- and 27-interception seasons). He’s also not showing signs of a player who appears mentally and physically done.

Former NFL quarterback Matt Hasselbeck played until he was 40, with his final three seasons as a backup in Indianapolis. Hasselbeck said there are usually signs a quarterback is on his way out. Are they still looking to improve in the offseason? Is the desire there? Is football what they love to do or is their golf handicap more important? It’s what Hasselbeck called life choices.

“It doesn’t have to be bad things,” he said. “Some guys are ready to say, 'Hey, had a good run' and move on.”

Manning doesn’t appear to fall into that category. He’s all-in on football and continues to be the first in the building and last to leave every day. He has been healthy, having never missed a start because of injury.

Other members of Manning’s 2004 draft class have shown signs of being closer to the end.

“[Manning] doesn’t at all. Ben Roethlisberger is in the opposite boat,” Hasselbeck said. “He’s talking about retirement; he doesn’t seem to ever have a season where this is not drama as a game-time decision, 'Will he play?' That is hard for a coaching staff to game plan. You don’t know if you have him Wednesday or you have somebody else.”

Manning’s commitment has never been questioned. He is a team captain, universally respected in the locker room and tasked with what one player recently described as “soooo much responsibility” because of what he’s asked to do on the field and at the line of scrimmage every single play. This is partly why the Giants coaches have confidence in their quarterback; he handles all the “other stuff” effortlessly.

Manning’s skills also don't appear to have diminished much. He has never been mobile, and his arm strength isn’t an immediate concern.

“I still see him throwing the ball really well down the field,” said veteran Giants linebacker Jonathan Casillas, who has also played with Brady and Brees.

Manning has insisted his arm strength and velocity are the same now as they were earlier in his career, in part because of a throwing program he adopted several years ago. The Giants think, despite the slip in numbers and the poor performance in the opener, they will be just fine.

Their quarterback will be just fine.

“I would say Eli, like everyone else on the offensive side of the ball, had a bad night, had a night that was not up to our standards,” Sullivan said. “Seeing his work ethic and how he prepares and seeing the things he’s done out on the practice field and the communication that is continuing to get better and better with the receivers and the protection adjustments and so forth, that it is one bad night.

“To rush to conclusions where he’s at or [where] anybody is after one game would be premature.”