Deontay Wilder suffered a bicep injury shortly before his defeat to Tyson Fury according to his sparring partner.
Junior Fa, who was part of the American's training camp, revealed the injury was sustained ahead of the showdown in Las Vegas back in February.
The American heavyweight's reign as WBC heavyweight champion was brought to a dramatic end when he was sensationally stopped by the Gypsy King.
Deontay Wilder's reign as WBC heavyweight champion was brought to an end by Tyson Fury
Junior Fa says Wilder received a bicep injury during training camp in the days before the fight
Fa told Sky Sports: 'I was very shocked by the Wilder . 'The lead up and the training was actually really good. Deontay was looking great.
'I think he did hurt himself towards the end of camp, which I don't think would have played too much into the fight, but then I don't really know the extent of the damage of the injury that he sustained.
'What I do know that he did get injured, but I was very surprised by Fury's game plan.
'As soon as the fight started, and I saw Fury not really taking a backward step, trying to push Wilder to the ropes, I was thinking 'Oh man, this is going to be a hard night for Wilder.'
Fa says he thought Wilder was in trouble during the early stages of the fight in February
'Fury just basically did what he said he was going to do, which was stop the biggest puncher in the world. That was a very, very good performance from him.'
Asked about Wilder's specific fitness issue, Fa said: 'Yes, the bicep injury. I don't know the extent of the damage, but he did hurt it.'
Wilder initially suggested that his ring walk costume, weighing 40lbs, had weakened him before the first bell, but has since hinted at further problems in the build-up.
Since the fight Wilder underwent secret surgery on the troublesome bicep.
A third fight is set to take place between Wilder and Fury though a date has yet to be announced.
