Eddie Hearn, promoter for IBF, IBO, WBA, WBO heavyweight champion Anthony Joshua, does not believe Oleksandr Usyk will ever become a full fledged heavyweight.

Joshua will defend his belts against Usyk, the mandatory challenger, on September 25 at Tottenham Hotspur Stadium.

In 2018, Usyk finished up his run at cruiserweight when he unified the entire weight class with a twelve round decision win over Murat Gassiev in the final round of the World Boxing Super Series.

The bout will Usyk's third in the heavyweight division.

Last year, Usyk picked up a tougher than expected decision win over dangerous veteran Derek Chisora.

Hearn explains that it's skill, not size, that makes Uysk dangerous.

“I think that Usyk will never be a fully fledged heavyweight – a real heavyweight in terms of size. But what he will always have is a huge amount of skill," Hearn said to Talk Sport.

“It takes time for people to bed into heavyweight, so put it this way, it would’ve been a lot easier fight for AJ a year-and-a-half ago. Now Usyk has fought a real heavyweight contender in Chisora, that will actually help him to start to understand what needs to be done. But AJ’s a different kind of beast. A good big guy will always beat a good small guy, so we’ll see.”

Despite the difference in size, Joshua is taking Usyk very seriously.

The two boxers captured gold medals in their respective weight classes in the 2012 Olympic games in London.

"I've got another challenger who's good, just as good. I have to be serious about him now. He will definitely step up, he will definitely take the fight. That's what I need - people that are serious," Joshua said.

"I would have loved to have boxed for the undisputed championship because it was a big fight for the public. We had everything set up. I have to move forward and not keep dwelling."