Don't get your hopes up when it comes to seeing Floyd Mayweather box Khabib ... 'cause the fight is NEVER HAPPENING -- so says ex-UFC star Brendan Schaub.
We got the UFC fighter-turned-MMA analyst out at LAX ... and dude wouldn't even let our photog finish his question about a possible Nurmagomedov-Floyd tilt.
"Stop it," Brendan tells TMZ Sports ... "It's never happening. It's never happening."
"It's not happening. EVER."
Why's Schaub so confident about that??
"Khabib's not a striker!