Tony Bellew says Anthony Joshua will need to show different qualities to beat Oleksandr Usyk when the two meet later this year.
Joshua is preparing to take on dangerous WBO mandatory challenger Usyk in September, with the Tottenham Hotspur Stadium the likely venue.
Joshua was all set to meet Tyson Fury to unite the heavyweight titles this summer but an arbitration court ruled that Fury should take a third fight with Deontay Wilder, which will be held in Las Vegas on July 24.
That left Joshua scrambling for an opponent. However, it did not take long for Usyk to emerge, with Matchroom promoter Eddie Hearn striking a deal with the Ukrainian.
A laughing Hearn said recently: "I trust Alexander Krassyuk [Usyk's promoter] a lot more than I do Bob Arum (futy's promoter). There's no ulterior motive here."
The 34-year-old Usyk will pose Joshua a different challenge. 'The Cat' is unbeaten in his 18 professional bouts to date and defeated Derek Chisora last October.
Chisora was beaten on a unanimous points decision after Usyk stepped up from cruiserweight for his second heavyweight bout. Despite his November defeat, Chisora was not very complimentary about Usyk.
Asked if the southpaw could mix it with the big guns in the division, he told Sky Sports: "No, because in the heavyweight game, you have to fight, not box."
Chisora, 37, thought his "pressure" and "body shots" caused Usyk problems, but ultimately the younger man was a comfortable winner on the cards (117-112, 115-113,115-113) at Wembley.
Bellew also doubts whether Usyk has the tools to topple the big guns at heavyweight and does not think he can get the better of Joshua.
The former heavyweight is a good friend of Joshua's, but having lost to Uysk in his final fight in November 2018, he is well versed on the 6ft 3in fighter.
Bellew was leading on the scorecards in Manchester when Usyk came on strong in the eighth round and stopped the Liverpool man. That defeat brought the curtain down on Bellew's impressive career.
"I know exactly how good Usyk is," Bellew told talkSPORT in June. "He's an amazing talent. Best fighter I ever fought. But I don't believe he's big enough and dangerous enough to cause AJ problems."
Usyk 'will not last long enough'
Bellew then corrected himself and stated that he thought Usyk may have the power, but believes Usyk will not "last long enough".
Bellew said: "Well, he's big enough to cause him problems, I don't think he'd last long enough to defeat him. But, he will cause problems while he's standing and he's [AJ] going to have to get rid of him. He'll put the pressure on, make him pay for every single thing he earns in there.
"AJ is the best athletic heavyweight I've seen in the last two decades. As an athlete, mate, he's unbelievable. He's an unbelievable specimen, he really is. The power he can move at, the speed he can move at, the dexterity; it's unbelievable.
"He's on another level to everybody else. He's just got to keep improving on that boxing IQ, which he is. He showed that against Andy Ruiz Jr in the rematch. How unbelievable he was.
"In the [Kubrat] Pulev fight, he went back a little bit and showed the old things he was doing and the way he broke down the big men with the speed that counts."
Bellew though insists Joshua will have to show different qualities and may have to take some shots.
"But then he's going to have to show something different against Oleksandr Usyk again. He's going to have to box at times, put pressure on and he'll have to get hit as well.
"So he'll have to show different things that he hasn't done in his career so far, but then again, he's always improving, always getting better and it's just fantastic to see," Bellew added.
Joshua will have a height, weight and reach advantage over Usyk. The Simferopol-born fighter will though have rounds in his favour, although the overwhelming majority of those have been at 200 pounds.
Haye and Holyfield
It's certainly not an impossible task for Usyk and both David Haye and Evander Holyfield have stepped up to heavyweight to become world champions.
Haye, who is now Chisora's promoter, toppled the giant Nikolai Valuev in 2009 to win his first heavyweight world title. The fight was billed as 'David vs Goliath' with the Russian holding almost a 100 pound advantage over Haye.
Haye though was much the quicker man and outboxed the much slower Valuev, sticking and moving to win with a majority decision.
He defended his WBA title twice before losing to Wladimir Klitschko in 2011.
Before that Holyfield had successfully stepped up to heavyweight. He took just seven fights to become the undisputed heavyweight world champion with a third-round knockout win over Buster Douglas in 1990.