The left-back confirmed he had signed with United this week after coming off contract at Aston Villa earlier this year.
He spent a few weeks on trial at United in February and did enough to land a deal.
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Plunkett, 17, made his debut as a starter in United U18s defeat to Sunderland late last month. He was on the bench for the final league game of the season last week in the win against Everton.
The defender is likely to stay with the U18s as Harry Amass is promoted permanently to the U21s next season.