Even club captain Gary Neville admitted afterwards he had never seen him before but said he was grateful for his stoppage-time goal that gave United their memorable 3-2 win.
The 17-year-old made his debut as a 61st-minute substitute for Nani. He looked impressive from his first touch before scoring the goal that took United back to the top of the Premier League table.
He was then booked for running into the crowd and hugging his father in delight and finished the afternoon being named man of the match.
TALENTSPOTTER PROFILE: Federico Macheda
He has been scoring regularly for United's youth and reserve teams since joining the club in September 2007 after developing through Lazio's youth system.
Born in Rome, he signed professional forms last August and is regarded as one of the most promising young prospects of Italian soccer.
Known as "Kiko", United's website says: "He is strong on the ball, has good positional sense in the box and his clinical finishing meant manager Alex Ferguson gave him a Champions League squad number midway through the 2008/09 campaign."
Macheda told Sky Sports News: "Scoring a goal like that - I really enjoyed it."
Asked what Ferguson said to him when he sent him on, he replied: "Play simple, just play. When I got the ball I just wanted to score."
United skipper Neville praised the youngster's performance.
"We are grateful to him -- the quality he showed at the end was unbelievable. I had not seen him before today but he's a great finisher and you'd want it to fall to him in the box."
Ferguson, though, kept it all in perspective.
Asked what he said to him afterwards, the United manager replied: "Well done. He'll need to keep his feet on the ground because he will get a lot of publicity over the next few days and he'll need to handle that."
