Canada's Adonis Stevenson is in a "critical condition" in hospital after losing his WBC and lineal light-heavyweight titles to Ukrainian Oleksandr Gvozdyk in Quebec City.
Stevenson, 41, who was boxing's oldest and longest-reigning champion, suffered an 11th-round knockout, slumping in the corner after a flurry of punches.
He was put on a stretcher, before being taken to hospital by ambulance.
Stevenson's promoter Yvon Michel said his boxer was in intensive care.
Unbeaten Gvozdyk, 31, told a news conference on Saturday night: "I talked to the people at the hospital. He was very confused when he arrived, we suspect a concussion. It's very disturbing."
On Sunday, Michel tweeted his thanks on behalf of Stevenson's wife and family: "Thank you for your many messages of encouragement and support. #Superman is currently under careful medical supervision of very competent doctors in a controlled environment. #hope."
The World Boxing Council tweeted: "The WBC and boxing community from the world join in prayers for the safe recovery of our dear friend, great fighter and exemplary human being @AdonisSuperman,"
The University Hospital of Quebec said the Haitian born boxer "was admitted to the emergency room" without giving further details, news agency AFP reports.
Stevenson was making his 10th title defence since winning the belt in 2013 by defeating American Chad Dawson.
He successfully defended the title against Britain's Tony Bellew.
Gvozdyk, who won an Olympic bronze medal for Ukraine at the London 2012 Games, claimed the belt in his first shot at a major title.