Anthony Joshua has been warned that he cannot repeat his performance against Jermaine Franklin when he takes on Francis Ngannou this week.
Johnny Nelson admits he is 'worried' for the Brit and says he could get knocked out by Ngannou if he is not on top of his game on Friday.
Joshua will enter the ring for the fourth time since last April when he takes on the former MMA star in Riyadh, Saudi Arabia.
The former two-time heavyweight champion is the big favourite heading into the fight against an opponent who only has one professional boxing match to his name.
Anthony Joshua has been warned he must be on his game against Francis Ngannou
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Nelson, who remains the longest reigning cruiserweight world champion of all time, initially thought Joshua would ease past Ngannou.
However, the closer it has got to the fight, he is now thinking twice about how it will go.
Joshua has attempted to get his heavyweight career back on track in the last year and has beaten Franklin, Robert Helenius and Otto Wallin.
The Brit was not convincing in the first of those three bouts against Franklin at the O2 Arena last April.
And Nelson says, having seen Ngannou up close, he does not think Joshua can afford to put in a repeat display 11 months later.
Asked about his prediction, Nelson told Sky Sports News: "I started with Anthony Joshua, day and night, shutting him out. It's not even a conversation.
"But when I'm seeing the demeanour, the model of these athletes, the mannerism of Ngannou who is not going to be pressured in any way shape or form.
"He wasn't even looking at AJ when he was in the ring, dismissed Tyson Fury - 'Look this is business'.
"I believe him. I've seen them all come, all shapes and sizes, from Tyson to guys that are journeymen.
"He's a great hustler or I believe it. And I've started to believe this guy.
"You know what, AJ, if you go in there and box like you boxed Franklin, you're getting knocked out.
"If you go in there and you doubt yourself one second with this guy, you're getting turned over."
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Johnny Nelson admits he is worried for Anthony Joshua against Francis Ngannou
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Nelson is still sticking with his gut feeling that Joshua will get the job done on Friday.
However, he admits he is now worried about the potential of a negative outcome for the 34-year-old.
Nelson added: "So I'm hoping that Anthony Joshua and Ben Davison, Ben Davison has done an amazing job, making him think 'I can do this job, I believe in what I can do'.
"But I'm struggling. I want to stay with it, so I'm not turning against AJ. I'm just saying 'I'm worried'."
Francis Ngannou is facing Anthony Joshua in just his second boxing fight
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Meanwhile, promoter Frank Warren admits Joshua is in 'real jeopardy' this week.
He told Sky Sports: "Will [Ngannou] learn from his last fight? Will he look a better fighter this time around?
"AJ's putting it all on the line here. He can't afford to lose this fight, having come off such a good win last time out.
"So this is real jeopardy for him, no doubt about it. It will be knockout chaos."
