Bare-knuckle boxer Nathan Decastro shared several gruesome photos of his face, which was absolutely smashed during a recent bout in London, and Nathan's pissed ... but not because he was hurt. He's livid the doctor stopped the fight, suspecting a broken orbital bone.

Decastro -- who holds a Bare Knuckle Boxing record of 2-1 -- ate several shots to his left eye during his fight against Lorawnt-t Nelson at the BYB 12 London Brawl event on Oct. 16.

The blows caused the 31-year-old's eye to swell up like a balloon ... and caused the ringside physician to call the fight at the end of the 3rd round.

The super middleweight fighter was furious Nelson took home the win ... and Decastro expressed his frustrations the next day with photos of his bloody injury, saying "I am f***ing devastated."

"I never complain. I just get on with each and every fight!" Decastro said on social media.

"That said the ref was poor I’ll not get into that as everyone can see it for themselves!"

"The doctor stopped the fight in the corner after the 3rd round because of my eye a suspected broken orbital which absolutely broke my heart."

Decastro -- a former British champion -- says he begged officials to let the fight continue (he believes he was winning the fight, despite the damage to his face), but to no avail, the fight was called off.

"I'm willing to go to the death," Decastro said, "but they stopped it against my will I begged to carry on, I would of win the fight but it was taken out of my hands!"