Dillian Whyte claims Anthony Joshua has lost his "killer instincts" and should stop boxing "if he doesn't want to do it anymore."
Joshua outclassed Jermaine Franklin at London's 02 Arena to win by unanimous decision on his return to the ring on Saturday night.
But Joshua had been expected to produce an explosive knockout and a fight that was billed as a "new dawn" for the 33-year-old left many fans unconvinced.
The two-time world champion had the nerves of a man fighting with his career on the line and was tentative rather than destructive in his comeback win.
And Whyte – who edged Franklin in a contentious points decision last November – has delivered a brutal assessment of his long-term rival.
READ MORE: Anthony Joshua silences critics with dramatic victory against Jermaine Franklin
"Joshua has lost all his aggression and killer instincts," Whyte, who was ringside, told BBC Radio 5 Live. "If he doesn't want to do it any more, I think he should stop."
Analysing Joshua's performance on DAZN, Whyte added: "I don't think he looked particularly good tonight. He seems a bit apprehensive, the right hand is still there and the boxing skills are still there, but the aggression is not there.
"The difference is in the last two rounds I went after Franklin, he didn't. 'I'll always have a go, win, lose or draw, I'll have a go."
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Joshua's lack of ruthlessness was also questioned by Otto Wallin, another top-10 fighter in the heavyweight division who wrote on Twitter that "he is not the same guy anymore" and "feeling too much pressure."
Whyte, meanwhile, was defeated by Joshua in 2016 during the Brit's rise to global superstardom and has been linked as his next opponent ahead of a possible summer bout.
But Joshua is already targeting a belated blockbuster clash with Tyson Fury and immediately called out the Gypsy King after recording his first victory in over two years.
Fury's talks for an undisputed fight against Oleksandr Usyk have broken down completely and Joshua now wants to capitalise on the certainly at the top of the division.
He told Fury: "Wherever you are, if you are listening, you know my management, you know my promoter, we've had dialogue before so let's continue this and hopefully we can get this done sooner rather than later."
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