Johnny Nelson has been ruled out of his WBO world cruiserweight title defence with Enzo Maccarinelli on 4 March after injuring knee ligaments in training.
Nelson spent the weekend in hospital
The British rivals were due to meet in Manchester as chief support for Joe Calzaghe's super middleweight unification bout with Jeff Lacy.
But Nelson was in hospital all weekend and has withdrawn on medical advice.
"I am absolutely gutted. I was really up for this fight and had trained exceptionally hard for it," he said.
"I can assure Enzo that he will be my next fight and also my incentive to get back to full fitness."
Swansea's Maccarinelli will still feature on the 4 March MEN Arena show in a WBO interim world title fight.
That could be against the man Nelson beat on a split decision in his last fight, Italian Vincenzo Cantatore.
"The Nelson news is very disappointing for everyone concerned," said promoter Frank Warren.
"It was a fight boxing fans up and down the country were looking forward to, me included.
"We are looking at a number of world-ranked opponents for Enzo, including Cantatore, and I'm hoping we can announce a replacement within the next couple of days."