Well, Paul just gave us the answer -- and it's about as gangster as it gets.

"This is the crazy stuff that will either set me up for a major fight and move my career forward or I'll prove that I got balls that are bigger than anybody else's in the division."

No lies detected.

Paul -- who hasn't fought since February -- stepped up to replace Islam Makhachev in the main event on Nov. 14 ... after Islam was forced to withdraw this week due to a staph infection.

Felder says UFC firmed up the fight on Monday night -- giving him less than a week to prep himself for war.

Fortunately, Felder says he's been training like a maniac recently for a triathlon -- which means his cardio is insane at the moment.

So, he weighed the pros and cons of fighting a legend like dos Anjos -- and quickly came to the decision that it's worth the risk.

"If he beats me, he's a former champion! I don't give a sh*t! It's Rafael dos Anjos!"

Felder continued, "I mean, I've got nothing to lose other than some brain cells and some broken bones and things as usual but I will heal up and I'll be fine."

Don't get it twisted, Paul says he wouldn't have taken the fight if he wasn't 100% confident he can win.

"I'm fully capable of beating him, that I will make everyone clear of! I'm not coming in here to get a payday."

Felder says there's a real chance this could be the last fight of his career -- but he's not worried about that. He's just getting himself ready to fight.

Dos Anjos is the former UFC lightweight champ -- with signature victories over stars like Nate Diaz, Robbie Lawler, Kevin Lee and "Cowboy" Cerrone.