Oleksandr Usyk will return stronger than ever from a fitness concern to eventually face Derek Chisora.
A problem with Usyk's elbow means his expected meeting with Chisora will be postponed.
March 28 was the initial date targeted but their fight is now likely to take place in May at The O2 in London.
Promoter Eddie Hearn told Sky Sports last week: "It looks like Usyk against Chisora is almost there. It's going to take place a few weeks later than anticipated."
It is a setback for the undefeated Usyk who chose to risk his WBO mandatory challenger status by fighting Chisora instead of waiting for his chance to face Anthony Joshua.
"Usyk will fight Joshua and this is a fact," his promoter Alexander Krassyuk, who works alongside Eddie Hearn, previously told Sky Sports. Krassyuk insisted on Tuesday night that Usyk's latest issue is not serious and will not significantly hamper his 2020 plans.
Joshua is expected to face his IBF mandatory challenger Kubrat Pulev in June in London, meaning Usyk would have to wait until the latter part of this year for his shot.
Usyk has endured problems with injuries before in the past few years.
His World Boxing Super Series final against Murat Gassiev in 2018, which he eventually won to become undisputed cruiserweight champion, was postponed due to an elbow problem.
Last year his scheduled heavyweight debut against Carlos Takam was cancelled when Usyk suffered a bicep injury. He eventually fought Chazz Witherspoon in October instead, which remains his most recent fight.
David Haye, the manager of Usyk's expected next opponent Chisora, remains one of just two men to become world champion at cruiserweight and heavyweight. Usyk could become the third.
Haye told Sky Sports last year about Usyk's chances: "He must stay injury-free."
Asked about the biggest difficulty in moving to the top weight class, Haye said: "The sparring. You have to condition yourself to heavyweight sparring.
"I used to bring in big heavyweights. It is different sparring cruiserweights to sparring heavyweights. It can break you down - you get hit on the shoulders, on the arms, there is a higher chance of injury because the impact is bigger.
"When you land your own shots? Punching someone who is 14st is very different to punching someone who is 19st. There is more pressure on your own hands. There are more bruises on your hands, more impact on your shoulders, more chance of damage."