Kenyon Martin played for the Nets and Knicks and was not reluctant to express his opinions during either experience. Now that he is 39 and a mere alumnus of both franchises, he is not about to get shy.

Asked on Sunday if he thinks Brooklyn can work as a viable long-term home for the Nets, he said, “It should. In Brooklyn, they appreciate good basketball. They appreciate if you lay it on the line each and every night.

“They appreciate good talent, and as of late, the Knicks and Nets haven’t had that. I’m saying both sides, the talent level hasn’t been there. So I think they’re cheating the fans, man. I really do think the fans of Brooklyn and over the way [in Manhattan], man, they’re getting cheated.

“Just my opinion, and it starts with management. It starts up top, those guys picking the talent. You need to hold those guys accountable.”

Martin was speaking at Barclays Center, a building in which he never played for the Nets. But he did help secure the two Eastern Conference championship banners from the early 2000s that hang there.

He just had appeared, briefly, for Trilogy on the opening day of the BIG3 three-on-three league, leaving the game after two minutes and two points because of a hamstring “tweak.”

Martin was the first overall pick in the 2000 NBA Draft and spent four seasons with the Nets. He played parts of two seasons for the Knicks earlier this decade.

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BIG3 recruited him in part for his name recognition and in part because his gift of gab helps with marketing and his rugged style and trash-talking fit what the league wants on the court.

“It’s real basketball,” he said after a quadrupleheader that attracted a crowd of 15,177. “They appreciate hard-nosed, tough basketball, and that’s what this is. They don’t need the glorious, everybody-shoots-35-threes-a-game league. That’s what the NBA is now. Some people like that style, some people don’t.

“We’re here to fill that void for the people that appreciate the way the game has been played forever. Basketball is a contact sport. They need to clean this up, of course, but basketball is a contact sport, and the [NBA] has taken all contact out of it.”

By “clean this up,” Martin meant a physical style that got out of hand at times Sunday. Overall, though, most involved seemed happy with the BIG3’s debut.

“It’s great, a great turnout, and I’m glad the fans got an opportunity to come out and support what we’re trying to do here,” he said. “It’s a good start. Overall play I think was good. It’s going to get better as the league goes over the next nine weeks.

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“There are a lot of guys who retire because of age or the league tells them they’re not good enough anymore — whatever the case may be. It just gives guys a different avenue, man, instead of going overseas.

“Guys have family and business ventures and other things that are here. So if you can come in and play basketball and pretty much make the same money you’re going to make overseas, why not stay here? It gives guys a chance to continue their careers with less strain on the body, but it’s still going to be competitive. For a true competitor and a true professional, this is the place for you.”

Martin received a warm ovation from fans at Barclays upon being introduced.

“I kind of made a name for myself in the area,” he said, “being on the other side of the river for four years and what we accomplished over there and then coming [to New York] for two seasons, they always have appreciated the way I play the game.

“So it’s a great, warm feeling for them to be able to come out and see me play again and see us play again. The warm reception was great.”