Unified heavyweight titlist Anthony Joshua's mandatory title defense on Oct. 28 against Kubrat Pulev is off after a right shoulder injury forced the Bulgarian challenger to withdraw. 

Joshua (19-0, 19 KOs), who unified the IBF and WBA titles in April by stopping former champion Wladimir Klitschko in their fight-of-the-year candidate, was looking to make his fourth title defense overall in the Showtime-televised fight at Principality Stadium in Cardiff, Wales. 

Veteran Carlos Takam (35-3-1) was tabbed as the late replacement by the IBF on Monday and Joshua promoter Eddie Hearn of Matchroom Sport quickly signed a deal, which had been discussed prior to training camp as a possible fallback. 

"I received a call from [Pulev promoter] Kalle Sauerland late afternoon to inform me that Pulev had injured his shoulder and maybe ruled out of the fight -- this was later confirmed by his doctor," Hearn told Sky Sports News. "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."

Takam, 36, a native of Cameroon who fights out of France, has suffered high-profile defeats against unbeaten WBO champion Joseph Parker and former secondary titleholder Alexander Povetkin.

"It's a difficult position for AJ 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 October 28."

The news comes just weeks after WBC titleholder Deontay Wilder (38-0, 37 KOs) saw his Nov. 4 title defense against Luis Ortiz fall apart when the unbeaten Cuban slugger tested positive for a banned substance. Wilder, 31, who has campaigned publicly for a unification fight with Joshua, will instead face mandatory challenger Bermane Stiverne (25-2-1, 21 KOs) in a rematch. 

Pulev (25-1,13 KOs) reportedly suffered an injury during training last week and kept it secret until it became clear he wouldn't be able to compete. The 36-year-old, who fights out of Germany, suffered his lone defeat by knockout to then-champion Klitschko in 2014.