It’s hard to keep good help.

The Nets lost a top assistant Wednesday, with Ime Udoka finalizing a deal to take over as head coach of the Celtics.

After just a single season with the Nets — who could lose two other assistants who are up for vacant head-coaching spots — Udoka will move on in a hire first reported by ESPN and confirmed by The Post. He’ll replace Brad Stevens, who moved up to president of basketball operations earlier this month replacing Danny Ainge.

The 43-year-old Udoka — who was long overdue for a head-coaching position — was essentially Steve Nash’s defensive coordinator. He’ll bring a no-nonsense, tough-love approach to the Celtics, after coaching that way in Brooklyn and also getting to know several of Boston’s stars at the 2019 FIBA World Cup.

“Ime focuses a lot about defense, because obviously when he played that’s basically what he was, a defender, and he liked to be physical,” Nets guard Mike James said. “Just for me personally, he’s always reminded me to do the little things: sprint back, load up, box out, be physical.

“He’s not like a coach that’ll hold back how he feels. He’s going to tell you how he feels, how you’re playing, what you should be doing better. For somebody like me, obviously that might not be the same what everybody else is doing. But I like coaches to be honest and tell me what they want me to do and it works out.”

Like much of the Nets organization — from GM Sean Marks on down — Udoka is part of the San Antonio coaching tree. After playing seven seasons in the NBA with the Knicks, Lakers, Blazers, Kings and Spurs, he joined Gregg Popovich’s staff as an assistant in 2012, winning a title in 2014.

Udoka had been a candidate for the head-coaching job in Brooklyn in 2016, before Marks — with whom he’d worked in San Antonio — went with Kenny Atkinson. He ended up getting hired as a lead assistant in Philadelphia in 2019, but when Brett Brown got fired after last season, he finally landed with the Nets.

Boston also had multiple interviews with Bucks assistant Darvin Ham and Clippers assistant Chauncey Billups. After Udoka had made an impression on Jayson Tatum, Jaylen Brown and Marcus Smart while coaching under Popovich at Team USA in 2019, ESPN reported their backing separated Udoka from the pack.

He may not be the only assistant the Nets lose in the coming weeks.

Mike D’Antoni is reportedly the front-runner in Portland. Jacque Vaughn has been granted permission to interview in New Orleans, where ex-Nets assistant GM Trajan Langdon is the general manager.

“We have incredible coaches, and they all are really worthy of being head coaches, or already have been. It will be [likely]. I’m assuming we will have turnover,” Nash said. “I’m assuming some of these guys will get jobs, because they’re that good and would really add a lot to an organization.

“We’re here to support them in that, and if they move on we want to make sure they’re in a place and a position they want to be in. And if we can help at all, they afforded us a lot this year with the strength of our staff, and we would be in a position where we’d have to replace a few.”

James Harden has withdrawn from the U.S. Olympic team, coming off his hamstring injury during the postseason, according to the Athletic. His Grade 2 strain cost him 18 straight games near the end of the season and essentially the first four games in the conference semifinals after a setback.

Nets-Bucks Game 7 was the second-most watched NBA game this season with 6.91 million viewers, behind only Lakers/Mavericks on Christmas Day.

Spencer Dinwiddie has been fully cleared for all basketball activities from his partially torn ACL, his operating surgeon Dr. Riley Williams told The Athletic. He has opted out of next season to become an unrestricted free agent.

“He looks and feels and moves like the pre-injury Spencer Dinwiddie,” Dr. Williams told The Athletic.