"The Diamond" has been getting treatment all week ... and judging by his recent pic, it ain't goin' too hot for the lightweight.

"Wut da helllll," Poirier said on Twitter ... alongside a shot of the deformed foot.

Poirier is hoping to finally get out of the hospital on Wednesday ... but made the best of his stay on Monday night, when he tuned in to his New Orleans Saints' matchup against the Tampa Bay Buccaneers.

Unfortunately for Poirier, it wasn't too fun of an experience -- his home team ended up losing in the final seconds as Tom Brady and Co. pulled off the wild comeback.

Poirier -- who beat Michael Chandler in November -- also took advantage of his time by talking trash with Conor McGregor ... saying he's down to step in the Octagon with him for a fourth time.