Adonis Stevenson has undergone surgery after suffering a "traumatic brain injury" against Ukrainian fighter Oleksandr Gvozdyk on Saturday.
The 41-year-old was knocked out in the 11th round of the light-heavyweight bout in Quebec City.
"Mr Stevenson underwent surgery on the night of Saturday to Sunday and has since been hospitalised in the intensive care unit," said Dr Alexis Turgeon, a specialist in intensive care medicine at the Enfant-Jesus Hospital of the CHU de Quebec-Universite Laval.
"He suffered a severe traumatic brain injury. Although critical, his situation remains stable under the circumstances.
"His condition requires mechanical respiratory support, deep sedation and specialised neurological monitoring. It is too early to comment on Mr Stevenson's long-term prognosis."
Stevenson was making the 10th defence of his WBC title at the Videotron Centre before a flurry on punches from Gvozdyk ended the contest.
A statement from the hospital added: "Mr Stevenson's family thanks supporters for their many messages of empathy and support over the past few days.
"Relatives want to be at his bedside and ask everyone to respect their privacy. As agreed with Mr Stevenson's family and friends, please note that no further comments will be made."
