The Cameroon goalkeeper has agreed to move to the Turks on-loan for the season, reports BBC Sport.
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United accepted Trabzonspor's offer for their goalkeeper last week.
There's a confidence Onana will be signed and settled in time for Trabzonspor's September 14 clash with Fenerbahce.
He will be immediately installed as Trabzonspor's No1 after they sold Ugurcan Cakir to Galatasaray for €36m over the weekend.
Onana's departure will see Senne Lammens establish himself immediately as United's new No1.