Jarrell Miller has been denied a licence to fight Anthony Joshua in New York on 1 June after an “adverse finding” in a drug-testing sample.
Briton Joshua, 29, was due to to defend his IBF, WBA and WBO heavyweight titles against undefeated American Miller, who has never fought for a world title.
A sample taken from Miller on 20 March prompted the decision by the New York State Athletic Commission.
Miller’s camp have now requested a B sample be tested.
If that sample comes back negative then Miller can make another application for a licence.
Joshua’s promoters Matchroom said Joshua will make his American debut on 1 June irrespective of Miller and have a “list” of alternative opponents.
Matchroom chief executive Frank Smith told BBC Sport: “We are working on a list behind the scenes and we will work tirelessly until we find a suitable solution.
“We have sold 15,000 tickets already here at Madison Square Garden, 7,000-8,000 of them to British fans. We have sold tickets to people from 50 different countries and they have booked flights, hotels and everything and you have to factor that in.
“Our plan right now is that we carry on with Madison Square Garden on 1 June and we will announce more details soon.”