American light-middleweight Patrick Day is in an "extremely critical condition" after undergoing emergency brain surgery following his defeat to Charles Conwell on Saturday.
Day was knocked out in the 10th and final round of the contest on the undercard of Oleksandr Usyk's heavyweight debut against Chazz Witherspoon at the Wintrust Arena in Chicago.
His promoter Lou DiBella said in a statement on his website: "Patrick Day suffered a traumatic brain injury during his bout on Saturday night.
"He was rushed to Northwestern Memorial hospital where he underwent emergency surgery. As of Sunday evening, Patrick is in a coma caused by the injury and is in extremely critical condition."
DiBella added on Twitter: "Pat Day makes any room he is in a better place. I've never met anyone who's met Patrick and not liked him.
"Never heard him utter a mean word. Never saw him greet someone without a big smile. Life doesn't seem fair sometimes. Please keep Pat in your prayers, thoughts, and hearts."
Conwell wrote on the social networking site: "My thoughts and deepest prayers are with Patrick Day and his family right now. His family will provide further information at the appropriate time. Please keep Pat in your prayers."
British promoter Eddie Hearn added: "Our thoughts and deepest prayers are with Patrick Day and his family right now.
"We have been in constant communication with his promoter @loudibella who will provide further information at the appropriate time. Please keep Pat in your thoughts."