Shadasia Green was treated for a brain bleed after her stoppage loss to Lani Daniels in New York.
Green was boxing Daniels, a former heavyweight titlist, for the unified IBF and WBO super-middleweight championships on the undercard of Alycia Baumgardner's clash with Bo Mi Re Shin at the Madison Square Garden Theater in New York.
A barrage of shots from Daniels at the start of the ninth round prompted the referee to pull Green out and she was taken from the ring in a stretcher.
Green posted on social media, saying: "I passed out due to bleeding out of my brain.
"However I am in ICU on the road to healing. I'll be back to get what's mine 100 per cent covered."
Green became a unified world champion when she handed Britain's Savannah Marshall an upset loss last year and she had become a prospective opponent for US star Claressa Shields, a multi-weight undisputed champion.
Green added: "With everything in me I want to thank those who had great concern and even the people who figured out during the fight something was wrong.
"I appreciate MVP [Most Valuable Promotions] and Nakisa [Bidarian, her promoter] being right by my side in the hospital."
Most Valuable Promotions also stated: "MVP is working closely with Shadasia and her family. Collectively we want to thank the New York State Athletic Commission, Madison Square Garden and the attending hospital for their leadership and care."
