DALLAS — The Nets knew exactly what kind of opportunity they squandered Saturday, knew just how much of an edge they’d wasted in catching the Hawks on the tail end of a back-to-back.

The next night in Dallas, it was the Nets laboring in leaden shoes. And the result got every bit as ugly as expected, the Nets getting run off the floor and routed 130-93 before 19,184 at American Airlines Center.

Looking sluggish and spent, the Nets got shredded on defense. They never led, fell behind by 37 and were noncompetitive the whole night.

“It’s control the controllables. If you play extremely hard and you make mistakes, you’re allowed to make mistakes; but when those mistakes are for not giving everything that you have physically and attention, that’s tough,” head coach Jordi Fernández said beforehand. “We cannot all be like, ‘Oh, my fault,’ and then have 12 ‘my faults’ or 10 ‘my faults.’ So we’ve got to be better. We’ve got to grow. I’m the first one that I’ve got to own my mistakes too. And just get better and grow as a team.”

They saw Cooper Flagg dominate with 25 points, 12 assists and five rebounds while former Net Kyrie Irving scored 21 against his former team.

Michael Porter Jr. led the Nets with 19 points and nine boards, but Julius Randle struggled with just 13 and finished a minus-33. This team isn’t capable of winning with that sort of performance from Randle.

Brooklyn allowed 57.1 percent shooting — including 61.9 percent in a first half that saw them trail 66-46 — and unlike the Hawks a day earlier in Atlanta, they never rallied.

The Nets had caught Atlanta on the tail end of a back-to-back and built a 17-point cushion. They even led by four with 1:45 to play before coughing up 10 unanswered points, letting the Hawks close the game on a 14-7 run.

Fernández said he hoped they could replicate their energy from the night before, but actually sustain it this time. Instead, apparently they had exhausted it.

“[I want to see] that this is a team that is ready to fight for 48 minutes,” Fernández had said beforehand. “We had some good things [Saturday] night, but overall, we couldn’t sustain the energy of the first quarter. And that’s against a team on the second night of a back-to-back. We got outplayed the whole game. That’s a good thing about the performance numbers: I know who runs harder, I know who runs longer and all those things.

“So, we need better effort. And sometimes also where there’s tough moments and disconnection, that’s where sometimes the energy goes down. And we have to overcome those moments together and with good energy. And I know, like I always say, the group works well in the day-to-day, they pay attention, they have suggestions, they talk, they work. So now we’re ready to put 48 minutes together.”

That didn’t happen. They barely put together four minutes — or eight. They shot 38.8 percent and 11-for-39 from deep.

Brooklyn fell behind by 13 in the first quarter, and that deficit doubled to 26 in the even sloppier second.

When Randle threw the ball behind Porter coming upcourt, it was a turnover on the sideline with 1:48 left in the half. The latter banged the ball on his head in frustration.

But the sequence that really typified the evening for Brooklyn?

Noah Clowney got whistled for a foul on Morez Johnson Jr., the two having to be separated with 3:51 left in the third quarter as the officials went to replay to see whether the Net forward had committed a flagrant. The replay showed that Johnson had actually brazenly knocked Clowney to the court on the other end of the floor moments earlier, so he was assessed a technical.

And then Moritz Wagner clanked the ensuing free throw with the Nets trailing 92-62.

Clowney promptly fouled out to salt the wound.