Anthony Joshua's upcoming fight against Kubrat Pulev in Cardiff is set to be cancelled, according to World Boxing News.
The Londoner hoped to defend his WBA 'Super' and IBF heavyweight world titles against mandatory challenger Pulev at the Principality Stadium.
More than 70,000 tickets had already been sold with a 78,000 full house expected on October 28.
It was set to be Joshua's return to the ring following his epic battle with Wladimir Klitschko at Wembley Stadium in April.
Bulgarian reports suggested Pulev had suffered a shoulder problem on October 6 but the 36-year-old was hoping to be fit regardless.
However, on Monday evening World Boxing News reported that Pulev has a partial pectoral tear and will be forced to withdraw.
He is reportedly looking to re-arrange the fight with Joshua in January 2018.
Joshua is hoping to fight the likes of Joseph Parker or Deontay Wilder in 2018 and Pulev's reported withdrawal could put the 28-year-old former Olympic champion in a problematic position.
He must complete his obligation to the IBF so Joshua will likely be hoping that a replacement can be found.
With just 12 days before the bout, boxing promoter Eddie Hearn - who was already planning Joshua's fights for 2018 - has a major headache.
He can either desperately look for a last-minute replacement or scrap the fight entirely.
"I think Anthony’s next fight after Pulev will be abroad. Maybe Las Vegas, maybe the Middle East," Hearn told the Mirror last week. "Then his next fight after that will be a unification — and I want the Wilder fight.
"It’s all there for Anthony, and he just has to keep winning. But first, let’s deal with the Pulev fight."
The two alternatives
Dillian Whyte - scheduled to fight Finland's Robert Helenius - is training for the undercard at the Principality Stadium but could conceivably take on Joshua instead.
His only ever defeat came against Joshua in 2015 in their fight for the British title.
Hearn's other option, according to WBN, is Carlos Takam.
Ranked number three by the IBF, Takam would be the preferred choice but convincing him to fight Joshua in Wales in just 12 days could prove rather difficult.
The 36 year-old Takam boasts an impressive record of 35 wins, three losses and a draw.