The unified world heavyweight champion is set to face the Ukraninian challenger at Tottenham Hotspur Stadium on September 25, and he'll be putting his WBO, WBA and IBF titles on the line.
Fomer boxer Bellew has faced Usyk before, hanging up his gloves after losing to him in 2018, and he's offered a word of warning to Joshua.
He told talkSPORT: "This will be the best boxer Anthony Joshua will ever face in his career.
"When it comes to natural, pure boxing, Oleksandr Usyk is the most natural boxer Anthony Joshua will ever face. He is unbelievable at what he does.
"He is not a true heavyweight.
"Everyone keeps going on, yes he is a cruiserweight, but he is the greatest cruiserweight who has ever put on gloves.
"So he does come with an amazing skillset, but I just think weight divisions exist in boxing for a reason."
However, Bellew admitted he's still backing Joshua to retain his championships as he pointed to his endurance.
He added: "Come rounds seven, eight and nine, he is going to find out why he is not a true heavyweight.
"I think Anthony Joshua’s speed, power and perseverance will be what catches up with Oleksandr Usyk.
"I wish he would stop fighting my friends because I want to support him!
"I’m a huge admirer of him and I have so much respect for the boy, but he just keeps fighting my friends so I can’t support him.”