Khabib Nurmagomedov broke his foot just three weeks before what proved to be his swansong against Justin Gaethje.
The Russian kept his injury quiet in the build-up to the fight which saw him choke out his challenger in the second round.
Nurmagomedov's coach Javier Mendez also revealed his fighter was suffering from mumps during his training camp.
Upon learning of the champion's retirement, UFC president Dana White said: "We are all lucky that we got to see him fight tonight because apparently he was in the hospital with a broken foot three weeks ago.
"I think he has two broken toes and a bone in his foot broken, that’s what his corner told me.
"He never told anyone, was just walking around as normal, he is one of the toughest human beings on the planet.
"He is the No1 pound-for-pound fighter in the world and you have to start talking about him having GOAT status with whoever else you think is the GOAT.
"When guys want to fight, the fight and when they don’t they don’t.
"You have got to give them time and let them heal mentally more than physically, with what he has gone through losing his father.
"We are all lucky to have seen him on Saturday night."
