Former UFC star Sage Northcutt has suffered the most devastating defeat of his career, a 29 second knockout in his much-anticipated One Championship debut.

The 23-year-old was left face first on the canvas after eating a huge punch from his opponent Cosmo Alexandre in the first round of the bout last week.

While Northcutt was standing up and alert after the fight, the damage he took proved to be extensive.

Northcutt took to social media to share an update of his condition, saying he underwent surgery for a remarkable nine hours to repair eight fractures in his face.

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"Fresh out of surgery...I had 8 fractures and a nine hour intense operation," he wrote on Instagram.

"Feeling blessed for the amazing care and all the support from @onechampionship, my family/friends and fans!"

It was Northcutt's first loss of his career via knockout.

"Super Sage" was touted as the next big UFC star during his time with the organisation but failed to deliver on performances.

He earned a 7-2 record before getting cut by the UFC in November last year, with president Dana White saying the young fighter needed to "get some work in some other organisations".