Anthony Joshua has said Jarrell Miller "does not deserve to be in a ring with me or any other heavyweight", after reported drug test failures.
Joshua, in a YouTube video uploaded on Monday, however, wished Miller the best for the future.
He also 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 champions US debut, but that fight was called off after the American boxer reportedly recorded adverse findings in drug testing samples.
Joshua 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."