Defending champion Dubois will enter the domestic dust-up as a huge underdog due to his lack of experience against the elite of the division.
The 26-year-old was stopped by Oleksandr Usyk in the ninth round of his first world title fight last August.
Former WBC cruiserweight king Tony Bellew reckons the same fate befall Triple D, but in even more devastating fashion.
The Bomber told Coin Poker: “I think Anthony Joshua knocks Daniel Dubois out inside three rounds.
“Everything that Daniel Dubois does well, Anthony Joshua does better.
“Daniel Dubois is a strong puncher, he’s heavy-handed but Anthony Joshua is stronger and faster.
“He got hit with 45 right hands in four rounds against Filip Hrgovic.
“He’ll get hit with three against Anthony Joshua and he’ll wake up in an ambulance. Anthony Joshua has dynamite in his gloves.
“Don’t get me wrong, Daniel Dubois is a puncher too, so he does have a puncher’s chance, but I rate it at 70-30, 80-20 in Joshua’s favour.”
Dubois knows he faces an uphill task to retain his belt and hand AJ only his fourth career defeat.
But he’s looking forward to the challenge and ushering in a new era of British heavyweight boxing.
He said: “That’s just where I’m aiming for, to fight the best and be the best.
“AJ’s been the king for a long time and on the night, I need to become a king slayer and that’s my goal, that’s the mission at hand.
“I’m learning more about myself as a fighter, as a person, coming out of the darkness and into the light, improving all around as a fighter and as an athlete.
“I’m up for this and ready to go.”
Joshua, 34, knows Dubois well having previously sparred with him and is refusing to underestimate his chances of winning.
He told Sky Sports: “I’m not saying he has the chance to do it on September 21, because sometimes what you speak comes into existence.
“But I won’t let it happen and I’ll show you how good I am.
“Whether it’s in the ring sparring or in the ring fighting, I won’t let anyone manhandle me or try and knock me out. No way.”
