The attacker struck four times against Norwich City as United won 6-4 on the day.
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Rooney made his debut with the U18s last month in their FA Youth Cup win against Derby County.
He posted to social media: "PL cup semi final. 6-4 win vs Norwich. Four (football emoji)."
He also shared an image of his No 7 shirt on the dressing room floor after the game.
Rooney is being offered scholarship forms by United after turning 16 and is expected to soon put pen to paper.
