The talented 16-year-old is one who the club rate very highly, per the Manchester Evening News.
Given he is able to sign a pro contract in the summer, United want to hold onto Mantato.
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The source adds that introducing him to first team training may be a way to convince him to stay.
Charlie McNeill, Dan Gore, Willy Kambwala and Omari Forson have all received debuts under current boss Erik ten Hag.
United have also used Alejandro Garnacho and Kobbie Mainoo, both teenagers, extensively this season.