Poirier was declared the winner via doctor's stoppage.

The fight was absolutely crazy ... but with only seconds left in the 1st, Conor attempted to plant his foot and his leg buckled above his left ankle/lower shin area. It was gruesome.

Conor -- who stayed in the Octagon and was interviewed by Joe Rogan while sitting against the fence -- said "this is not over. If I have to take this outside, I'll take this outside."

Poirier had to be held back from charging McGregor as CM talked to Joe.

Despite what McGregor said, with a serious injury like this ... you have to wonder if we'll ever see Conor back in the Octagon.

As for Dustin -- who was billi strutting around the Octagon -- it's clear even after the win and Conor's injury, their beef is still very much alive.

Poirier speculated McGregor broke the bone earlier in the fight while throwing a leg kick that DP checked ... something Conor denies.

Khabib Nurmagomedov weighed in on social media, saying ... "Good always defeats evil. Very happy for @DustinPoirier"

"I hope you will get the belt end of the year" ... Eagle wrote.

Floyd Mayweather also took a jab at Conor ... Tweeting a photo of a $50K betting slip, writing, "Thanks for getting me paid!"

The crowd was almost as electric as the fights ... some of the biggest names in Hollywood were in the house -- from President Donald Trump, Justin Bieber, Megan Fox, MGK, Kourtney Kardashian and Travis Barker, Kevin Durant and more showing up for the bout.

KK and TB decided to put on a show as everyone got into fight mode -- straight-up making out as the camera panned to their area.

Cleveland Browns superstars Baker Mayfield, Austin Hooper and David Njoku also showed face ... with the QB sitting right next to UFC legend Chuck Liddell -- but yeah, they did NOT follow in Kourt's footsteps.