Fury claimed a split decision win over Ngannou in Saudi Arabia despite being knocked down in round three.
One judge scored it 95-94 in favour of Ngannou but two gave it to Fury at 96-93 and 95-94.
Ngannou was making his professional boxing debut and is in line for another big-money fight.
Fury's win sets up a blockbuster bout with Oleksandr Usyk and Hearn wants Ngannou to fight Joshua.
Hearn told Boxing Social: “Now Francis Ngannou has beaten Tyson Fury, I’m up for it.
“I haven’t spoken to AJ. I have spoken to a few people [in Saudi Arabia]. I think Fury vs Usyk is a damp squib, but if it’s contracted and it’s for undisputed… undisputed fights are always great. But I’m not interested in that fight. I’m interested in Fury vs Ngannou rematch… I have to speak to the boss…. but I like AJ vs Ngannou. For me, light work.
“I’m not going to say he’s a world-class heavyweight but you have to respect that he just beat Tyson Fury. What should have happened is some random MMA fighter who has never boxed before just became the lineal heavyweight champion.
“It should have been one of the most incredible stories in the history of heavyweight boxing. Everyone thought Ngannou won.”