Chazz Witherspoon has replaced Tyrone Spong as Oleksandr Usyk's opponent for his heavyweight debut in Chicago on Saturday.
Spong was denied a boxing licence by the Illinois State Athletic Commission after there were adverse findings in two of his urine tests, according to Matchroom Boxing.
The 34-year-old, however, issued a staunch denial, telling BoxingScene.com: "This is a set up.
"I vehemently deny that I have ever taken the substance named in VADA's [the Voluntary Anti-Doping Agency] report.
"I have asked for testing of the B sample at a laboratory of my choosing. Testing performed last week at the request of the Illinois Boxing Commission showed, what I've always known, that I'm a clean fighter."
The bout will be Usyk's long-awaited heavyweight bow, the Ukrainian having moved up after dominating at cruiserweight. He was initially set to face Carlos Takam in May but had to withdraw due to injury.
Usyk will now finally return to action against Witherspoon, who possess a 38-3 record with 29 knockouts. His last defeat came to Seth Mitchell in April 2012.
Witherspoon said: "I can't wait to face Usyk in Chicago. I have been in training ready for a big fight, and it doesn't get bigger than this. Oleksandr is stepping up to heavyweight – and he's going to find out that it's a totally different game.
"I've won my last eight fights and I really feel that I've been waiting in the wings for a huge opportunity like this and I am going to put every ounce of myself into the fight on this massive stage."