Starsport understand the Red Devils have bid £75m for the Belgian striker - and he’s set to have a medical in the USA.
United were interested in Real Madrid striker Alvaro Morata but a transfer has failed to materialise.
Boxing sensation Bellew, who is also a lifelong Toffees fan, reckons Ronald Koeman will struggle to replace Lukaku.
"I'd be heartbroken to see him leave, he's a fantastic player," Bellew said on Sky Sports.
"He's scored tonnes of goals from inside the box, outside the box.
"Romelu Lukaku is irreplaceable, you're not going to get one man who's going to come in and score the same amount of goals that he did. It would take three guys to chip in."
Everton coughed up over £90m on five players this summer.
Jordan Pickford, Davy Klaasen and Michael Keane have all been signed.
Despite Lukaku’s impending exit, Bellew insists his boyhood team are heading in the right direction.
"But saying that, we've spent an awful lot of money on an awful lot of proven quality, so let's just see how it goes," he added.
"I'm happy with how Everton's doing business, it's great. The chairman's doing great things, I know the club are working very, very hard behind the scenes.
"Even with things like ground improvements and in the transfer market there's been massive goals that have been set and achieved.
"Everton is a club that's on the up, we are coming back and getting back to where we belong and that is near the top."
