Sage Northcutt Has Surgery for 8 Fractures After ONE MMA Knockout

Sage Northcutt was destroyed way more than anyone thought after a 29-second, first-round knockout in his ONE Championship debut -- turns out he suffered 8 fractures and was in surgery for 9 hours to repair the damage.

Northcutt posted a horrific pic of the aftermath ... hooked up to all sorts of stuff post-op in the hospital.

Sage's debut was a disaster ... the 23-year-old took a devastating punch at the hands of Cosmo Alexandre to the face, sending him to the canvas with a thunderous face plant. Clearly, the punch did damage, but the face plant may have exacerbated things.

He posted the hospital pic, saying, "Fresh out of surgery ... I had 8 fractures and a nine hour intense operation. Feeling blessed for the amazing care and all the support from ONE championship, my family/friends and fans!"

It's surprising ... because Sage got up under his own power and stood there as the ref raised Alexandre's hand and declared him the winner.

Sage was cut from the UFC after a lackluster run ... UFC honcho Dana White said Northcutt just didn't cut muster and needed to work on his game.