Alexander Povetkin has tested positive for coronavirus so his rematch with Dillian Whyte has been postponed.
Whyte was set to seek revenge for a knockout defeat against his Russian adversary on November 21 but he must now wait until a targeted date of January 30 - a further announcement will be made about the status of the show on November 21.
Whyte's promoter Eddie Hearn immediately suggested that he could step in to challenge Tyson Fury instead.
Fury has shelved a trilogy fight against Deontay Wilder and instead announced his first WBC heavyweight championship defence on December 5, but hasn't confirmed an opponent yet.
Whyte and Fury now find themselves ready for action within a fortnight of each other, but neither has an opponent.
Whyte lost his status as the mandatory challenger to Fury's WBC title when, after flooring Povetkin twice, he succumbed to a dramatic uppercut.
Derek Chisora claimed on social media that he would step in to fight Whyte for the third time on November 21, just three weeks after losing to Oleksandr Usyk: "I would take that fight. No bruises, ready to rock and roll."
Hearn reacted to the news of Whyte's postponed fight: "Firstly we want to wish Alexander Povetkin a speedy recovery.
"This is a challenging time for shows. There will be lots of ups and downs over the next few months. We look forward to the fight happening in late January."
Povetkin's promoter Andrey Ryabinskiy said: "The Povetkin-Whyte rematch has been postponed. Povetkin fell ill with a coronavirus infection and is now under the supervision of doctors in one of the Moscow clinics, taking care of his health.
"The course of the disease is normal: there is a fever, but there are no critical complications. We will hope that he will be able to recover quickly enough, but we think that the fight will only take place next year.
"There is no talk about changing the opponent at the moment. Povetkin will box with Whyte".
Hearn had previously told Sky Sports about Whyte's hunger to avenge Povetkin: "If he wants to fight for a world title, everything is on the line in this fight.
"In that sense, everything is on the line in his career.
"It was a big shock especially because he was controlling the fight. It was a brutal knockout.
"Povetkin said: 'I'm all-in, I knocked you out once and I'll do it again'.
"Whyte is raring to go. I've never seen him so motivated to win a fight.
"The bad boy in Whyte is coming out for this fight. Sometimes when fighters create success and gain wealth, do they become softer? I've seen another side to Whyte.
"This is the old Dillian Whyte - that bad guy, that street guy who wants to do a job on Povetkin.
"This time, it's strictly business in an absolute must-win fight."