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Forget legacy, Conor McGregor should quit fighting and retire already ... so says UFC star Derrick Lewis.

The Black Beast joined the guys on the "TMZ Sports" TV show (airs nightly on FS1) and said Conor doesn't have anything to prove anymore and he's made so much money fighting, he should step away from the Octagon and enjoy the rest of his life.

"He made life-changing money," Lewis says ... "He really don't have to fight."

"I'm fighting just so I can take care of my family. I don't fight just because I love it just that much. I'm fighting because it's a job for me."

In fact, Lewis says if he made a $50 million payday for a fight like Conor, he'd cut off everyone and disappear.

"I'll probably even get a divorce then," Lewis joked.