The former heavyweight world champion will return to the ring against Kristian Prenga on July 25, in what will be his first fight since his training team members and childhood friends Sina Ghami and Latif Ayodele tragically passed away in an accident in Nigeria last December, and his resilience has been praised by the promoter.
Hearn told Sky Sports: "They're billing this fight as the comeback - he only fought six months ago, but this is one of the greatest comebacks, because of where he was physically, mentally, emotionally.
"He stepped up to the plate and he never shied away from it.
"He comes back with so much vengeance and emotion because he does it for Sina and 'Latz' Ayodele. The strength and the support that he's given to the family, is absolute priority to him.
"Every day he's in there training harder than you can imagine in this training camp to achieve his dreams. We need to celebrate great individuals, great athletes, but also people with a never-say-die attitude, like Anthony, who's got it all.
"He's got all the money in the world. He's a two-time world heavyweight champion. Yet he's over there [in Spain], away from everybody, in isolation, training three times a day till he can't train any more.
"He wants it more now than he's ever wanted it before."
Hearn explained that Joshua's "top priority" is to help the grieving families of Ghami and Ayodele.
The Matchroom chief said: "There is absolutely no doubt those two great men [Ghami and Ayodele] would have wanted AJ to come back even stronger through this.
"AJ's top priority is to protect and support those people. Probably a priority over the fight and everything and, like I said, he has stepped up to that plate. Even when the accident happened, even when the injuries had occurred, the only thing on his mind was to be there for other people.
"What people didn't see was not just the emotional pain and the mental pain, but the physical pain that he was in from that accident. It was only two or three weeks after that, he was back in rehab.
"He wants it more now than when he didn't have anything. Before Olympic championships, before World Championship silvers, when he was just in Finchley and he was dreaming one day of bettering his life and making money and becoming a world champion.
"He wants it more now when he's got everything. What does that tell you about the individual? It tells you he's a true competitor - but you've got to be good enough.
"Now he's got to go in there and he's got to brutalise [Kristian] Prenga on July 25. We've got to go into that Fury fight with momentum and that aggression and that volatility to go and do a job on Tyson Fury."
