AUBURN HILLS, Mich. — Knicks legend Walt Frazier sees once-confident Iman Shumpert now looking as if he’s not having fun.

Shumpert, who was often compared to Frazier during his rookie season because of his defense-first approach and quick hands, has fallen onto the trading block, and Frazier sees it affecting him.

“He’s always been a very confident guy,’’ Frazier told The Post before the Knicks’ 92-86 loss to the Pistons. “I think he just has to find his role. Sometimes he doesn’t look like he’s happy out here. Maybe that’s because he heard the rumors he’s being traded. But he’s very stern in his face. It doesn’t look like he’s having fun.’’

Frazier, whose documentary on his Atlanta upbringing debuted Tuesday night on MSG Network, said he could see why Knicks president Steve Mills would look to see what he can get for Shumpert, who scored 11 points and grabbed two rebounds against the Pistons.

“You look at team, he’s a valuable asset,’’ Frazier said. “One of the few guys they can trade. Not surprised by that.’’

“It has to be a breakout year for him. He was there as a rookie. The injury set him back, but he came back strong last year. This has to be the year he shows something.’’

As for the documentary, it was highlighted by Frazier getting emotional when talking about the segregation he grew up with in the South.

“It was my motivation, brought up like that,’’ Frazier said. “It was in my work ethic. You can’t rest on your laurels. If I scored 30 points [in high school], the next day my coach would bring me the paper where a white guy scored 40. I still had to work harder. That was the kind of way where you can never rest on what you were doing. [People] never knew how good I was because I was never in the white papers, only in the black papers.’’

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Kenyon Martin has gone from a platoon system, playing every other game with a minutes restriction to starting center with no-holds barred.

Martin, who started at center down the stretch last season when the Knicks reeled off 13 straight wins, started against the Pistons and scored five points and grabbed seven rebounds in 29 minutes.

Martin, one of the grittier players in the league, said he thinks he can help the Knicks’ woeful starts defensively, and as long as his ankle holds up, could remain the starting center until Tyson Chandler (broken leg) returns in three-to-four weeks.

“We got to be more aggressive on the defensive end,’’ Martin said. “We realize we haven’t gotten off to a good start as a team. I’m hoping I can add that spark.’’

Martin can’t believe effort has been an issue with the team.

“For that to be a conversation around here, it’s a shame,’’ Martin said. “You can’t teach effort. It has to be a part of you.’’

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Coach Mike Woodson said he is hoping Metta World Peace, who missed his second straight game Tuesday, returns Wednesday against the Pacers. Woodson confirmed World Peace had fluid drained from his left knee Monday.

Asked if he’s worried because World Peace had surgery on the knee last season, Woodson said, “There’s a lot of concerns right now. Again we got to coach the guys able to play.’’