Boxer Patrick Day is in an "extremely critical condition" after his defeat by Charles Conwell in Chicago.
The 27-year-old American is in a coma with "a traumatic brain injury", said his promoter.
Day was taken to Northwestern Memorial Hospital after being knocked out in the 10th round of his super welterweight bout on the undercard of the Oleksandr Usyk-Chazz Witherspoon fight in the US.
"Pat makes any room he is in a better place," said promoter Lou DiBella.
"I've never met anyone who's met Patrick and not liked him. Never heard him utter a mean word. Never seen him greet someone without a big smile. Life doesn't seem fair sometimes.
"Please keep Pat in your prayers, thoughts and hearts."
Day was taken from the ring on a stretcher on Saturday.
British boxing promoter Eddie Hearn wrote on social media: "Deepest prayers with Patrick right now."
His company, Matchroom Boxing, which organised the event, also posted to say Day had "been taken directly to hospital".
Speaking after the fight, 21-year-old American Conwell told IFL TV: "I give my blessings and concerns to him and his team. I hope he is OK. I know he is a great person."