Boxer Patrick Day was in "very critical condition" after his defeat by Charles Conwell in Chicago.
The 27-year-old American was in a coma with "traumatic brain injury," his promoter said.
Day was taken to Northwestern Memorial Hospital after being eliminated in the 10th round of his super welterweight class under the card from the Witherspoon Oleksandr Usyk-Chazz fight in the US.
"Pat makes any room in a better place," promoter Lou DiBella said.
"I've never met anyone who met Patrick and didn't like him. Never heard him say cruel words. Never saw him greet someone without a big smile. Sometimes life seems unfair.
"Please keep Pat in your prayers, mind and heart."
The day is taken from the ring on a stretcher on Saturday.
British boxing promoter Eddie Hearn write on social media: "The deepest prayer with Patrick now."
His company, Matchroom Boxing, which organizes the event, also posted to say Hari has been "taken straight to the hospital".
Speaking after the fight, American Conwell is 21 years old to IFL TV: "I give my blessings and concerns to him and his team. I hope he is fine. I know he is a great person."