It is over and done with, a first game from hell. Now the Giants must make sure their season doesn’t start tumbling away from them the way their 2015 debut against the Cowboys did.

“There’s no self-pity,’’ left guard Justin Pugh said. “It’s, ‘So what, now what?’ We got 15 games to go.’’

Game No. 2 is Sunday against the Falcons, the home opener at MetLife Stadium, with the Giants already 0-1 with one esteem-shaking loss on the ledger. The response to the 27-26 defeat, that included a late blown lead and mental breakdowns by Eli Manning and the coaching staff, will be revealed by how the Giants perform, playing for the first time in front of their frustrated fans.

“It really comes down to what the team is going to do,’’ Odell Beckham Jr. said. “The fans can cheer as loud as they want, and if the team’s out there not executing, it doesn’t matter how loud they’re cheering. Of course it’s great to have them there and have them in full support. It’s great to see everybody there and it looks like the stands are full and everybody is into the game. Just give them something to feed off of.’’

Manning faced the music this past week, chronicling his mental blunders in the closing minutes even as those around him pleaded to move on. In the past, he has been able to shake off a nightmarish performance with something substantially better.

“I’ve seen it in him many times before,’’ coach Tom Coughlin said. “It’s a discipline, just like anything else is a discipline. He’s better at it than most, to be honest with you. He’s probably better at it than I am.’’

Falcons LB Vic Beasley Jr. vs. Giants LT Ereck Flowers

This is an all-rookie battle. Beasley was taken with the No. 8 pick out of Clemson and Flowers went one pick later, No. 9, out of Miami. It’s a classic small-and-quick vs. big-and-strong confrontation.

Beasley is really a linebacker at 246 pounds, and Flowers (6-foot-6, 329 pounds) has more than 80 pounds on him. Flowers greatly impressed coach Tom Coughlin by refusing to miss time because of a sprained ankle, and Flowers will need all his mobility to deal with the speed-rush of Beasley — who was brought in to upgrade what was minimal pressure in 2014 (22 sacks for the Falcons, 31st in the league). The Falcons did not have a sack last week but they did put eight quarterback hits on Sam Bradford in a 26-24 victory over the Eagles.

DOWNFIELD BLUES: A down-the-field passing attack has been a Giants staple since Eli Manning took control more than a decade ago, but there was none of it in the opener. Manning never threw the ball deep and did not complete a pass of more than 19 yards to any of his receivers. Attribute much of this to the Cowboys defense, which plays two safeties deep and in the middle of the field. The Falcons play a single-high safety that should lead to more chances to get the ball down the field.

“So there should be more opportunity for us to make some plays down the field and pretty much show what we can do a little bit,’’ receiver Reuben Randle said. “We got to be prepared to win our one-on-one battles and make some plays for Eli. Try to carve ’em up a little bit.’’

THE ATLANTA SEAHAWKS: Manning said he not only studied tape of the Falcons on defense but also went back to last season’s Giants-Seahawks game to get a sense what the Falcons will bring to town this weekend. Their first-year head coach, Dan Quinn, was the highly successful defensive coordinator for the Seahawks, and he has installed that scheme in Atlanta. It is a 4-3 front — the Falcons in recent years were in a 3-4 — based more on execution than deception. Of course, Quinn does not have the talent with the Falcons that he had in Seattle.

“They’re disciplined, they’re physical, they run to the ball very well, they’re opportunistic,’’ Coughlin said. “They do more within the concept of how they play than you think they do. That’s where you’ve got to prepare yourself, because not everything that you see in the first game is inclusive on what they bring to the table, as they advance their defensive scheme.’’

THE DEFENSE RESTS: Manning’s mental blunders never would have surfaced if the Giants had not caved in on defense in the fourth quarter, allowing two ridiculously easy touchdown drives (six plays, 76 yards in 2:53 and six plays, 72 yards in 1:27).

“Now we’ve just got to learn from it and lick our wounds and move forward,’’ defensive end Robert Ayers said.

The Giants hope their talented cornerback tandem of Prince Amukamara and Dominique Rodgers-Cromartie at least can slow down beast receiver Julio Jones and their suspect pass rush can get some heat on Matt Ryan, after not laying a glove on Tony Romo.

“We need to go out there and redeem ourselves,’’ defensive end Cullen Jenkins said. “Not everything is gonna be perfect, adversity is part of the game. We have to respond. The key to this game is how you respond when you’re down. We were down after last game, we lost, so now it’s about how we respond.’’

TARGET PRACTICE: This is going to be a weekly development, at least until Victor Cruz returns and is able to establish himself as a legitimate threat. Odell Beckham Jr., coming off a record-breaking rookie year, is going to attract extra attention and the Giants are going to have to get creative to free him up.

“It’s not going to be easy to get Odell the ball,’’ offensive coordinator Ben McAdoo said. “He’s a guy that can change the game in a blink of an eye. Teams have a plan for him. So we need to do a good job of playing the shell game with him, so to speak. We can’t force the ball to him.’’

Beckham had five receptions for just 44 yards in the opener, and patience is not his strongest virtue.

“It’s something I’ve had to deal with in the past in my college career, things aren’t all gonna be there right away,’’ Beckham said. “You just got to wait for those moments when your opportunity is gonna be there and you got to do whatever you can to help the team.’’

Crossroads and crisis points come earlier and earlier. The Giants are confronted with both, and it’s only Week 2. They sound as if they’ve put the Debacle in Dallas behind them, and it would be very Eli Manning-like to have a big game in response. It remains to be seen if the Giants have any answers on defense other than their starting corners. The home crowd will get restless at the first sign of struggle.

Giants 27, Falcons 20