TMZ Sports talked to the 33-year-old #1 ranked UFC lightweight -- the guy who almost everyone believes will fight for the belt -- about McGregor's recent social media post.

If ya missed it, Conor fired off a message ... "So what date am I fighting Oliveira?" shortly after the UFC 269 fight between Oliveira and Dustin Poirier.

"I'll burn it all down, dude! What do you mean? F***ing burn it down, that's what I'd do," Gaethje told us when we asked how he'd react to Conor (#9 ranked) leapfrogging him.

Justin continued ... "[Conor's] so irrelevant. He has not won a fight since 2016 in the lightweight division. I'm ranked number one."

But, how about this ... if Gaethje wins, would he want to defend his strap vs. McGregor ... who he's been lowkey beefing with for years?

Nope.

"I would absolutely not allow him fight for the title. He did not win to go through the process. He's not gonna s*** on everything that we -- there's too many. Beneil Dariush, Islam Makhachev. Those are the guys that we fight. Not Conor McGregor. That's not how it works. He's becoming more and more irrelevant as time goes on. Pretty soon you won't be even asking me these questions. It's gonna be a beautiful f***ing day."

It's pretty cool ... Justin says he's finally on the doorstep of the moment he's been waiting for since he got into fighting.

"My whole life I've been working towards this. That is what's next. If not, I f***ing burn it down. It doesn't matter. I'll show up and start throwing dollies everywhere."