Ronaldo's pre-recorded the message was played to the Under-18s in the dressing room ahead of emerging onto the pitch for the final against Forest, says the Manchester Evening News.
United's youngsters were playing in front of an FA Youth Cup record crowd of more than 67,000 people but the video message from a player so many idolise was a major motivational tool for Travis Binnion's team.
Several of the squad have had training sessions with the first team, learning from the quality and dedication of Ronaldo, while he has also helped mentor exciting 17-year-old winger Alejandro Garnacho.
Garnacho scored scored twice yesterday for the 3-1 win.
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