Mike Tyson's manager, Shelley Finkel, claims his fighter suffered a serious knee injury in his loss to England's Danny Williams in Louisville.
Finkel said an MRI scan showed that Tyson had torn a ligament in his left knee in the opening round.
"That's why he couldn't throw the right hand the rest of the fight," Finkel said, adding that Tyson could not walk after the fight.
"I was screaming at him to throw it, but he couldn't."
Tyson dominated the first round, rocking Williams with shots to the head, before apparently motioning towards his knee and grimacing.
Williams then gained control and, in the fourth round, knocked Tyson out with a series of punches that left the former world champion sprawled on the canvas.
"In retrospect I wish he had said something and said he couldn't continue," Finkel said.
After the fight, Tyson's trainer, Freddie Roach, credited Williams with fighting a good fight, but said Tyson was hampered by the injury.
"It's kind of hard to pivot and throw some shots without your left knee," Roach said. "But Danny Williams surprised a lot of us."