The 16-year-old has still to make his Empoli debut but is regarded as a genuine talent and long-term prospect for the English champions.
United have been criticised in the past for signing young Italian talent before they can be secured to contracts by their respective clubs, as was the case with Federico Macheda and Giuseppe Rossi.
Massacci, if reports are to be believed, has already had a medical with United and is to fly over to Manchester on Sunday to thrash out the finer details of the deal.
"When Sir Alex Ferguson rang me up, I couldn't believe it," he told the Gazzetta dello Sport.
"I had an hour-long conversation with him, in English. He seemed a humble person and nobody from the club made the pressure of an historic side weigh on my shoulders.
"In life a chance like this only comes past you once, so you've got to grab it with both hands. It isn't an easy decision aged 16 to leave home, family and friends, but I don't want to have regrets."