Ivaylo Gotzev, co-promoter for heavyweight contender Kubrat Pulev, says the Bulgarian boxer fought off serious symptoms connected to the COVID-19 virus in the weeks leading up to his December mandatory fight with IBF, IBO, WBA, WBO champion Anthony Joshua.
Pulev was dropped three times before the contest was finally waved off in the ninth round at the SSE Arena in Wembley.
It marked only the second defeat of Pulev's career - with Wladimir Klitschko being his other conqueror.
Pulev was initially scheduled to fight Joshua in 2017 - but he withdrew with a shoulder injury. He was replaced by Carlos Takam, who Joshua stopped in ten rounds.
It took two years for Pulev to get back in a mandatory position.
Gotzev explains that Pulev, 39-years-old, was too worried about withdrawing for a second time - believing that he would never get himself back in a mandatory spot for a third time.
Pulev decided to go ahead, but he weakened as the fight played out.
"He didn't really have the power or the strength, nor did he have the sparring that he needed in preparation. Once again, not excuses, just facts, and that's what happened," Gotzev told Sky Sports.
"He says, 'No, I'm going through with it, I'm going to fight AJ, and I know I might never get a third chance at this fight ever again. It was too much to lose, not fighting for all those belts. He really believed in himself. He had a shot at cracking through AJ and he tried.
"Within a month prior to the fight, he had COVID. He had to alter his training and quite honestly I thought we were going home. Me to the US, him to Bulgaria. He met with doctors and he made a decision. He is standing by his decision. He's a warrior.
"Look, we've always given AJ credit, and I'm giving him credit now, saying he's a great athletic specimen. He's fit, he's physically strong, he can punch, but the weaknesses are obvious. There were glimpses of that when AJ was getting frustrated every time that Kubrat got close and punched him. Kubrat just didn't have enough. It does take your strength. Kubrat, his X-rays showed he went through a pneumonia stage, but he fought it off and he was there for the fight."