Giants quarterback Eli Manning sits on the sideline against the...

Giants quarterback Eli Manning sits on the sideline against the Cowboys on Sept. 10, 2017, in Arlington, Texas. Credit: AP / Michael Ainsworth

ARLINGTON, Texas — Every fear that Giants fans had about the offense during the offseason, every demon that passed through their nightmares, every concern that lurked around the corner for a team glowing in promise and optimism, they all became a stark reality Sunday night.

Yes, they were without Odell Beckham Jr., who was inactive with an ankle sprain. But the additions during the past months should have made that sting a little less painful.

They brought in Brandon Marshall. They drafted Evan Engram. They signed Rhett Ellison. And what did they get for a return?

Three points.

The Giants lost to the Cowboys, 19-3, the most disappointing opening-game performance by a Giants offense since the 35-0 loss to the Cowboys that began the 1995 season.

Read it one more time.

Three points.

Instead of being retooled, they regressed.

“You go into every game confident that your game plan is going to work and everything is going to run smoothly like you practice it,” Sterling Shepard said. “But sometimes it doesn’t go like that.”

Many will point to the one area in which the Giants did not make any offseason changes, the offensive line, as the cause for the loss. The line certainly played a role, as Manning never seemed comfortable in the pocket and was sacked three times. But the entire group fizzled out and wasted a decent defensive performance.

The Giants couldn’t even complete a pass to Marshall until the final play of the game with 13 seconds remaining, making him a non-factor in his Big Blue debut.

“The offense was very disappointing,” Giants coach Ben McAdoo said. “No part of the offense was functional tonight.”

Guard Justin Pugh put it more concisely: “We got our butts kicked tonight.”

Not on defense they didn’t. The Giants allowed one touchdown, a 12-yard pass from Dak Prescott to Jason Witten late in the second quarter, and four field goals. Ideally, that would have been good enough for a win. On Sunday night, though, they would have had to pitch a shutout to get the “W.’’

It put the defense in a difficult position.

“We expect them to score,” safety Landon Collins said of the offense. “We have a lot of keys, a lot of things that we need from the offense.”

Points being chief among them.

“I think the whole offense needs to make improvements, starting with me,” said Manning (29-for-38 for 220 yards with an interception). “I’ve got to be better prepared to make this team better.”

The night began with the team’s best player attempting to get on the field. Beckham, who sprained his ankle on Aug. 21, worked out with teammates during warmups but never seemed to reach top speed while running routes and catching passes from Manning. The Giants eventually decided to sit him. It was something they had been preparing for during the past three weeks.

“Obviously, he’s a tremendous player,” Manning said, “but we have players and we have to play better than that.”

Remarkably enough, the defense was able to keep the game within reach for the Giants. Their last gasp came with 7:48 remaining when Manning tried to hit Roger Lewis Jr. on a slant from the Giants’ 41. Anthony Brown picked off the pass and the Cowboys gobbled up the next 5:53 before Dan Bailey kicked his fourth field goal to make it 19-3.

While many of the offensive woes seem predictable in retrospect, or at least similar to last year’s issues, the Giants themselves appeared to be blindsided by them. They had spent the past month and a half of the preseason talking optimistically about the alterations they were making and improvements they were showing.

Asked if, even without Beckham on the field, he could have imagined the Giants being held to three points, Shepard shook his head.

“No,” he said. “But that’s what happened.”

So they go back to work to get ready for next week’s home opener against the Lions on Monday night. They can take comfort in knowing that the loser of the Giants-Cowboys opener last year wound up winning the NFC East title. They can reflect on how three of their four previous Super Bowl teams began their season with a loss.

It’s just one loss, right?

“It feels worse than that,” cornerback Dominique Rodgers-Cromartie said. “The Cowboys. A division opponent. That one hurts.”

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