Former IBO welterweight champion Chris van Heerden knows firsthand what it's like to BOX Conor McGregor -- and tells TMZ Sports the guy is gonna get ROCKED by Floyd Mayweather.
The 29-year-old professional fighter stepped in the ring with Conor in May 2016 to spar for a few rounds -- and dominated the UFC star.
And to rub salt in the wound, van Heerden says he was out of shape and had just gotten back from vacation when around the time of the sesh .