The goalkeeper will swiftly sign from Inter Milan to alllow him to join United on their US tour.
Dean Henderson is then expected to be sold to Nottingham Forest, with David de Gea already leaving the club.
De Gea left Old Trafford after 12 years after at the club when he failed to agree a new deal.
Manager Erik ten Hag identified Onana as the club's primary transfer target and will get his man.
The pair know each other well having worked together at Dutch club Ajax.
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Onana has already agreed personal terms ahead of a move to Old Trafford.
Man United's pre-season campaign has already started with a 2-0 win over Leeds in Oslo.
The Cameroon goalkeeper will join up with his new teammates for the US leg of their preparations.
They will face Arsenal in New York on Friday before games against Wrexham, Real Madrid and Borussia Dortmund.
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They will round off their preparations with games against Lens at Old Trafford and Athletic Bilbao in Dublin.