"I've never met a man who's been less impressed by becoming heavyweight champion of the world," said Hearn, after seeing Anthony Joshua dismantle Charles Martin in two rounds and take possession of the IBF title.
Promoter Hearn seemed almost put out - 20,000 screaming fans at London's O2 Arena, a reported £1m purse with a promise of many millions more to come and an opponent who turned up in fancy dress. What more did the British fighter want?
He even seemed irritated that his new belt kept falling over. "He's not used to those yet," said Hearn. On the evidence of Saturday's fight, he soon will be.
"I just don't want to make a big deal out of it," said the 26-year-old Londoner, having just won a world heavyweight title in only his 16th professional fight.
"I knew what I was here to do. I knew I wasn't losing. I spoke to someone on the phone the other day and said: 'Is it bad that I'm thinking about my post-fight speech already?'
"So what do you expect? When I retire on top I'll be happy."
History suggests Joshua is adopting the right stance. When American legend Mike Tyson beat Trevor Berbick in 1986 to secure his first world heavyweight title, fame and adulation had already spoiled him. By Tyson's own admission, he was riddled with gonorrhoea that night. No wonder he looked so angry.
Joshua disposed of Martin 63 seconds faster than Tyson disposed of Berbick. Joshua won a world title 12 fights faster than Tyson. Fingers crossed Joshua's career takes far longer to unravel - and that he's not spotted driving around with a tiger in his passenger seat any time soon.
"When was the last time in heavyweight boxing we had a role model young children want to be like?" said Hearn. "At Monday's public workout, he stayed for an hour and a half to sign autographs. He did another hour after the weigh-in and another hour after the fight. That's how he feels fans should be treated.
"If your son said, 'I want to be like Anthony Joshua', you'd be pleased with that. If he said, 'I want to be like Tyson Fury', what would you say?"
The laughter from the press pack told Hearn everything he needed to know.
But Hearn isn't stupid. He wants Fury to carry on being naughtier than ever so that, when the time comes, he can promote the fight as good versus bad, sensible versus daft - however you want to frame it.
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And anyway, Joshua isn't as nice as all that. For one, he's got a right hand that could fell an American fridge-freezer, never mind Charles Martin. And he knows full well that the British public wants a little bit more than a megawatt smile, some humble patter and the odd appearance on a panel show.
When Hearn was informed of Fury's assessment of Joshua's latest victory - Manchester's WBA and WBO champion called his rival "slow and ponderous" - the promoter, as promoters do, sounded indignant. But Joshua - who has plenty of the street about him, don't worry about that - gently steamed.
"Why should I lower myself to Tyson Fury's level? He should act like a champion, but he acts like a kid. You see what an eight-pack and some muscles can do, knock out a champion in two rounds. Maybe he should take some tips.
"I'm here to knock people out. That's what people want to see. People come to see blood and I've got no problem drawing blood for people. I enjoy it."
Suddenly, the heavyweight division, for so long so lukewarm, is beginning to bubble again: Joshua clattering through the ranks like a bowling ball through skittles; Fury with his ring cunning and air of malevolent mischief, affected or not; David Haye on the comeback trail; WBC champion Deontay Wilder, 'The Trap from Tuscaloosa' who can also hit a bit, holding up the American end.
"I'm not saying Fury or Haye are next, but they're certainly on the radar," said Hearn. "We're not going to wait two or three years to fight them. The pressure will be on us [from the British public] after a voluntary defence or two.
"And it's about time we cracked the American market. The big money is in the UK nowadays but we'd like some more money from our friends in America. The plan is to turn AJ into a pay-per-view fighter over there. Showtime and HBO both texted me within two seconds of Joshua's fight, saying: 'Let's talk.'"
Hearn also confirmed that the first battle between Joshua and Fury could take place at opposite ends of the country. Hearn said he had Wembley Stadium booked for Joshua's next fight on 9 July before Fury announced his rematch with Wladimir Klitschko would take place on the same date in Manchester.
Both men fighting different opponents on the same night seems like madness. But madness is Fury's stock in trade, while Hearn appears to revel in it.
"Sky turned down the Fury-Klitschko fight [which will be on subscription boxing channel BoxNation] because they didn't want to make a decision until Joshua had fought. They see AJ as the future," said Hearn.
"Sky are keen to go head to head with Fury-Klitschko. Fury did well beating Klitschko but it was an awful fight. When you see AJ fight, you get value for money."
Who knows how that little bit of nonsense will be solved. Sky might see Joshua as the future, but fighting a relative unknown when Fury is fighting a legend on the same night seems foolhardy, however bad the first fight between Fury and Klitschko was.
And with New Zealand prospect Joseph Parker fighting Carlos Takam in May, Bulgaria's Kubrat Pulev fighting Britain's Dereck Chisora in the same month and Cuba's Luis Ortiz fighting Alexander Ustinov this summer, there aren't many decent names available.
But rest assured, Joshua is up for pretty much anyone. He'd fight King Kong, if only the British Boxing Board of Control would grant him a licence.
"Me, Fury, Haye, Wilder, we have to fight, it's bound to happen. We can't go through this whole era dodging each other. That would be silly."
Far sillier things have happened in boxing, but Joshua at least sounds like he means it. And even if Joshua doesn't seem too excited about the ride ahead, you will be.
A few years ago, I asked Nigel Benn whether he thought Chris Eubank Jr would struggle to bear the burden of his famous name.
"It's very worrying because he has a lot to live up to," said Benn, a former two-weight world champion who fought two classic matches against Chris Eubank Sr in the 1990s.
"Chris Sr was a great fighter, so he's going to have so much pressure on him. If he jumps into the ring like his dad and has the same music, then he better perform like his dad."
So it was intriguing to see Benn's son, Conor, make his professional debut on Saturday. Decked out all in black like his dad. Boxing out of the same wide crouch as his dad. Exhibiting the same jerky head movement as his dad. Scrapping like his dad. Anything the Eubanks can do...
Eddie Hearn sounded like a dad who had organised the world's best birthday party, only for his son to declare afterwards: "Yeah, it was all right I s'pose…"