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 sh*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 sh*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.