The Nets finally got their leading scorer back, but even Cam Thomas’ buckets couldn’t save them from their own sieve-like defense Thursday night.

Brooklyn couldn’t keep Mark Williams off the glass, couldn’t guard Terry Rozier or anybody else on the perimeter, and suffered a humbling 129-128 loss to the horrible Hornets before a crowd of 16,072 at Barclays Center.

When Cam Johnson missed a wide-open potential winner in the final seconds, the Nets had wasted a great return from Thomas (26 points) and flushed a golden opportunity to win a season-high fourth straight.

“As clean [a look] as I could have asked for,” said Johnson, adding of his infuriated reaction “Just frustration.”

He wasn’t the only one.

Even perpetually positive coach Jacque Vaughn was vexed with this one, as well he should be.

Facing a Hornets team that had dropped six of eight and was without LaMelo Ball and Brandon Miller, this should’ve been a laugher. Instead Charlotte was the team smiling, after the Nets let Williams snatch 12 rebounds — five offensive — and Rozier light them up for 37 points.

This wasn’t a talent issue. It was a focus issue.

“It’s a concentration issue,” Vaughn said. “At the end of the day, defensive rebounding has not been a problem for us. When you don’t concentrate, then it becomes a problem. Without a doubt, Mark Williams was on our pregame tape, on our shootaround tape, all of the above, of how he can impact the game.

“There’s a list of threats, whether that is transition offense, whether that is Mark Williams rebounding for them, whether that’s them having uncontested open 3s from the dudes that we don’t want to. So whether that’s playing with a physicality and dictating on offense, some things that we did not do that we discussed that we said we were going to do.”

Mikal Bridges had 22 points, nine boards and four assists for the Nets.

And he gave them a 120-117 lead on a driving layup with 3:52 to play.

But the Nets allowed six unanswered and never led again.

“Yeah, credit to them — they executed well, made tough shots,” said Thomas. “Terry made tough shots. The kid off the bench made shots, Nick Smith Jr. hit some tough shots. And obviously they had some offensive rebounds that lead to fouls, Mark Williams with his putbacks and offensive rebounds that lead to 3s. So we’ve just got to clean it up.”

Rozier capped the run with a pull-up 3, to put the Nets in a 123-120 hole.

Spencer Dinwiddie pulled them within 127-126 on a turnaround bank shot with 57 seconds to play.

Rozier answered with a step-back with 39.9 seconds left, but Nic Claxton (20 points, 14 rebounds, two blocks) made a put-back layup 10 seconds later.

When Rozier missed a 20-foot step-back with 8 seconds to play and Dorian Finney-Smith rebounded the Nets had a chance.

But with Thomas sitting down the stretch, Johnson missed the potential winner.

Fittingly, it was Williams that got the rebound, after he’d punished them.

Both Vaughn and Thomas said his rebounding led to Charlotte going 21 of 36 from deep and might have been more costly.

“Yeah, because obviously it just leads to a whole bunch of things. It can lead to a foul or just an easy dunk,” Thomas said. “Honestly we’d rather live with the contested middie over Mark Williams getting an uncontested dunk that could lead to an and-one or whatever. So I feel like you gotta clean it up a little bit for next game.”

Thomas, on a minutes restriction in his first game back after sitting nearly three weeks, was on the bench late.

“Everyone knew going into the game minutes restriction. I’m not gonna say what that is, but good to have him back,” Vaughn said.

“It was just a minutes restriction, obviously. Obviously, you want to be out there, that’s the competitor in me. But whatever the plan is, that’s the plan. Maybe it’ll change, maybe it won’t, but I’m fine,” said Thomas. “We got a great look at the end. CJ just missed. It was a great look — wide-open 3. We’ll live with that all day, so I’m fine. I’m still on the plan so it’s all good.”