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 ... and still "landed at will" on McGregor.
We asked straight-up if Conor has the knockout power to beat Floyd, van Heerden's response -- "No."
But just because he doesn't believe on Conor doesn't mean he won't get into business with him -- Chris says he wants to fight on the Mayweather v. McGregor undercard ... and he even names his opponent, Lamont Peterson.
Fun fact -- that's 'Purge' star Frank Grillo sitting next to Chris. Turns out, the actor also helps manage and train the fighter. Cool, right?