Tony Bellew has been speaking after Fury announced his retirement on Twitter on Monday before reversing his decision three hours later, insisting he was "here to stay".
Fury is the reigning WBO and WBA heavyweight champion.
He was tweeting after more damaging headlines the week before reported that he had tested positive for cocaine, just a week after he withdrew from a rematch with Wladimir Klitschko because he was unfit to fight.
Fury's camp have not commented on the cocaine allegations.
Bellew, WBC cruiserweight champion, said he thinks Fury is struggling with mental health issues.
He said he is concerned for his welfare: "I think it is very, very sad, he has got a mental illness, without a shadow of a doubt.
"I know Tyson and his father and his uncle. They are some very good people, who I know, honest men, straight people.
"I am not condoning what Tyson Fury has done. If this cocaine thing is true, then it is absolutely disgusting what he has done. He deserves a long ban."
He went on: "His wife loves him, his children need him, I just hope that he comes home safe and does nothing silly because in the frame of mind he is in at the minute, he's not stable and they are worrying times."
Speaking about Fury's Twitter account he said: "He will look back at this when he grows up and he will be ashamed of himself. But ultimately we need to put that on the back burner and get him what he needs, which is help. Don't snarl or turn your nose up to people with mental health problems."
