Ex-NBA hooper Matt Barnes was one of the toughest defenders of his time ... and he's telling TMZ Sports how he'd handle the task of guarding Zion Williamson -- "Foul the s**t outta him!!!"
Barnes is retired now but when we saw him out at Delilah in L.A., he had some advice for anyone going up against the 19-year-old New Orleans Pelicans phenom.
"I'd have to foul the s**t outta him," Barnes said ... "He's too big, man. I'd have to body him. [That's how I'd] try to stop him. He's a big boy."
We also asked Barnes about Zion being officially listed in the NBA at 6'6" -- instead of the 6'7" height he was listed at as a freshman at Duke -- and Matt says at this point, one stupid inch won't make a difference.
"You see how dominant he plays, so height doesn't really matter."
Barnes points Zion is the same size as Charles Barkley ... and he turned out just fine!!!