Andy Ruiz Jr will face Anthony Joshua on June 1 following Jarrell Miller’s failed drug test.
Joshua had been scheduled to defend his IBF, WBA and WBO heavyweight titles against Miller at Madison Square Garden in New York.
That bout was thrown into doubt when Vada reported an “adverse finding” in a sample collected from the American heavyweight in March, denying him a licence to fight the undefeated world champion.
It was later revealed that Miller had failed a second test, with the 30-year-old later admitting on Instagram he had “messed up” and “made a bad call” as the fight was called off.
Joshua’s promoter Eddie Hearn and his team immediately began the search for a new opponent for that date, with the likes of Luis Ortiz, Kubrat Pulev, Mahmoud Charr, Trevor Bryan, Michael Hunter and Adam Kownacki among the names said to be on Matchroom's shortlist.
However, it is one-time world title challenger Ruiz Jr - AKA 'The Destroyer' - who has now been tasked with stopping Joshua with just under six weeks’ notice.
The 29-year-old Californian's professional record reads 32 wins from 33 fights, with his solitary defeat coming at the hands of New Zealander Joseph Parker - who was outpointed by Joshua last year - when the two fought for the vacant WBO strap in December 2016.
His last fight came only 10 days ago, when he dominated Alexander Dimitrenko and forced the German to retire before the start of the fifth round on the Danny Garcia-Adrian Granados undercard at Dignity Health Sports Park in Carson, California.
