Anthony Joshua will defend his IBF and WBA heavyweight world titles against Carlos Takam after Kubrat Pulev withdrew through injury.
The bout is scheduled for 28 October at Cardiff's Principality Stadium, with more than 70,000 tickets already sold.
It will be Joshua's first fight since stopping Wladimir Klitschko at Wembley Stadium in April, adding the WBA title to the IBF belt he claimed in 2016.
Pulev suffered a shoulder injury in sparring, with Takam the next in line.
The Cameroonian-French 36-year-old had been on standby for the fight, promoter Eddie Hearn said.
"I received a call from [promoter] Kalle Sauerland late afternoon to inform me that Pulev had injured his shoulder and may be ruled out of the fight - this was later confirmed by his doctor," he added.
"IBF rules state that the mandatory will go to the next fighter in line, which is Carlos Takam.
"When the Pulev fight was announced I made a deal with Takam's team to begin camp and be on standby for this fight. When I called them this evening they were overjoyed and good to go."
He added that the change of opponent put Joshua in a "difficult position".
"Having prepared meticulously for the style and height of Pulev, he now faces a completely different style and challenge in Takam," Hearn said.
"This hasn't happened in his career before but he is ready for all comers on 28 October."
Speaking in September, Hearn said Joshua's aim was to face Pulev before meeting Cuban Luis Ortiz, the mandatory challenger for his WBA title.
He also stressed that Joshua does not want to vacate either of his belts and would rather defend them before hopefully facing WBC title holder Deontay Wilder of the United States in the summer of 2018.
The heavyweight division's fourth recognised title is held by New Zealand's Joseph Parker, who retained the WBO belt with a points victory over Briton Hughie Fury last month.
BBC Radio 5 live boxing analyst Steve Bunce.
There is big pressure on Carlos Takam but, in theory at least, he has been training for this fight for the past two months.
His people had been told there was an outside chance of him facing Joshua, if anything happened with Pulev, back in August.
So if we believe that he has been preparing all that time, after his last fight in June, then at least we have a fit Takam.
He is only about 6ft1in. He fights small, about five or six inches shorter than Joshua, which doesn't look good.
But at least the fight is going ahead and rest assured, far worse fighters than Takam have challenged for world titles over the past 20 years.