Diaz stormed out of the UFC media event on Friday while Dana White announced the Conor McGregor comeback fight against Khabib Nurmagomedov ... and we got him outside.

Nate claims he's been ineligible to fight due to a legal dispute with his former manager. As we previously reported, The Ballengee Group sued Diaz, claiming he never paid them their cut for negotiating his deal for the McGregor fight at UFC 202.

This seems to be the case Diaz is referring to.

Diaz is upset, claiming White has been "bad-mouthing" him by saying he's turned down multiple fight offers over the past 2 years. Nate insists he's "never turned down a fight in my life."

Diaz says he finally settled his lawsuit last month and can fight again -- which is why he agreed to fight Dustin Poirier at UFC 230 in November.

But, something at the media event rubbed Diaz the wrong way and now he tells TMZ Sports ... the UFC "better start acting right and start over-promoting [me] instead of under promoting."

There's more ... we also asked Nate who he's got in the Conor vs. Khabib match -- and he replied, "I whipped one's ass and I slapped the f*ck out of the other one so they're both my little bitches."

Diaz then told us about an incident in which he smacked Khabib in the face in front of "all his Russian friends" and they "didn't do sh*t."