Jarrell Miller is being "nailed to the cross" over a failed drug test "because he is an African-American fighter", his promoter has claimed.
Sarah Fina, of Greg Cohen Promotions, said treatment of Miller differed to that suffered by other fighters.
Miller has apologised after an "adverse finding" saw his June fight against Britain's Anthony Joshua called off.
He was due to meet the IBF, WBA and WBO heavyweight champion Joshua, 29, at Madison Square Garden on 1 June.
The 30-year-old was denied a licence for the New York bout after analysis of a sample taken on 20 March.
Then after a second adverse finding was reported on Friday, the 30-year-old American admitted he had "messed up".
Some critics have since responded by saying Miller should now be banned for life.
"Firstly, I do not condone cheating whatsoever," Fina posted on social media.
"Am I upset and disappointed with Jarrell? Absolutely. What Jarrell has done is wrong 100%. But what I won't do is turn my back on him. People make bad choices in life.
"With all of that being said, it is hard for me not to think that he is being nailed to the cross because he is an African-American fighter."
She pointed out Mexican fighter Saul 'Canelo' Alvarez recently signed a $365m (£280m) multi-fight deal with streaming broadcaster DAZN, despite a six-month ban for failing two drug tests. He claimed to have eaten tainted meat.
"Even the very fighter that was asked to replace Miller, Luis Ortiz, tested for banned substances," Fina added.
Cuban Ortiz has been linked with taking Miller's place, though a replacement will not be named until next week.
The 40-year-old tested positive for banned substances in 2017 but avoided a ban because boxing chiefs found he took the drugs for high blood pressure.
Fina said she she had also suffered abuse over the issue for being Miller's promoter.
"I can't help but to think that a lot of that has to do with the fact that I am a woman in a male-dominated industry. This is not me playing the victim, but stating facts," she added.