It's true ... at least that's according to Calvin Johnson himself, who tells TMZ Sports he's that good with a club in his hands!!!

The former Detroit Lions superstar is adamant he could make a real run at earning cash full-time at hitting fairways and greens in regulation ... if only he had just a little bit more free time on his hands.

"If I had the time to put the level of work that I did when I played football, I think I would be able to get there," the Hall of Famer said. "Because I love the game. I hit more than my share of shots every time I go out. And it's like, 'You know what, that's like the pros do.'"

The problem for Johnson, though, is he simply doesn't have the hours to devote to swinging his irons and woods all day long.

"I'm married," the 38-year-old said. "I've got family, I've got a business now -- I don't have the time."

But don't cry for Johnson, he's killing it in retirement regardless. His Primitiv business just got a partnership with the Detroit Lions -- and he was greeted with a huge ovation at the NFL's draft in Detroit last week.