His doctor at the Quebec University Hospital said the boxer had undergone surgery soon after being admitted.
"He suffers from severe traumatic brain injury," Alexis Turgeon told reporters.
"His situation is still stable under the circumstances, but critical."
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Stevenson was knocked out in the 11th round when his Ukrainian opponent Oleksandr Gvozdyk delivered a vicious combination to end the Canadian’s unbeaten streak of 17 bouts.
Stevenson slumped to the canvas and though he eventually got to his feet, doctors deemed his condition serious enough to send him to hospital.
"His condition requires mechanical respiratory assistance, deep sedation and specialized neurological monitoring. It is too early to comment on Mr. Stevenson’s long-term prognosis," Turgeon said.
"It’s a traumatic brain injury. The majority of people come out of it with after-effects."
(Reporting by Andrew Both in Cary, North Carolina, editing by Ed Osmond)
