Anthony Joshua says Jarrell Miller "does not deserve to be in a ring with me or any other heavyweight" after reported drug test failures.
But Joshua has wished Miller the best for the future and confirmed that he hopes to have a new opponent for June 1 within the next "48 to 72 hours".
Miller was due to face Joshua for the world champion's US debut but that fight is off after the American reportedly recorded adverse findings in drug testing samples.
Miler has admitted that he 'messed up' in a post on Instagram - and Joshua posted a video on YouTube on Monday night which included a message for his planned opponent.
He said: "I don't want to talk too much about the situation with Jarrell Miller, because it is not in my character to knock a man when he is down.
"But there are a lot of things he has said previously, at press conferences and in the build-up - and I wanted to crack him in his jaw.
"But I know when you are in a position, you have to lead by example, and I knew that was not the right thing to do.
"But the lesson to learn is that karma works in various ways - what goes around will come back around.
"I feel he has taken fate and his own blessings out of his own hands.
"Sometimes be respectful - I know it is a fight - but be respectful, be appreciative and lead by example. What he has done is not lead by example.
"Cheats will get found out and hustlers and hard workers will always rise to the top.
"Good luck to Jarrell Miller with whatever he does in his life - but he does not deserve to be in a ring with me or any other heavyweight right now.
"I don't know what the future holds, that is not up to me, that is up to the governing bodies but they will make the right call."
Meanwhile, boxing fans are eagerly awaiting news of who Joshua will tackle at Madison Square Garden on June 1.
"We are currently looking for alternative options," said Joshua. "I have got another four and a bit weeks of sparring.
"Whoever the fighter may be, I am going to keep my chin down, hands up and I am ready to slug it out with whoever.
"Eddie (Hearn, promoter) is doing the negotiations and hopefully, within the next 48 hours, 72 hours, we will have an update for June 1st.
"It is still going ahead - and the landlord will come and collect his rent!"