The Red Devils' line-up was circulated the night before Sunday's 2-1 victory away to Manchester City.
Amorim's team selection proved to be exactly the same as the leaked line-up, with both Marcus Rashford and Alejandro Garnacho dropped.
The Portuguese has now started an investigation to find the source of the leak, according to The Sun.
Both Garnacho and Amad Diallo have been questioned by the United boss in a bid to find the player who has been leaking the information.
Diallo produced a man of the match display against City at the Etihad Stadium.
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The winger won the penalty for United's equaliser before scoring a dramatic last minute winner.
Meanwhile, Garnacho's brother Roberto has previously been accused of leaking team news.
Line-ups have been circulated in the past when the Argentine international was absent from the squad.
However, Roberto denied being the mole last week.
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Amorim also addressed the situation on Sunday, explaining: "I know that story, I think it is impossible to fix, you have a lot of people in the club, the players talk with agents, it is not a good thing."
Despite his team selection being leaked, Amorim was victorious in his first Manchester derby.
Late strikes from Bruno Fernandes and Diallo secured a comeback victory for the Red Devils.
Although they remain in 13th place, the result cuts the gap to both their city rivals and the top four.
United are now five points behind fifth place City and a further point away from the Champions League spots.
"I think we did a great job, we controlled it very well," the former Sporting Lisbon boss said.
"We didn't allow much, of course we don't create so much either but it was a divided game. But we believed a lot.
"It was different from Arsenal, we controlled the Arsenal game in the first half but I didn't feel that belief. Today I felt it and I think that was the key point to change the game.
"I think for the fans it has a deep meaning especially in this context, this moment, but we had a difficult moment one week ago against Nottingham (Forest) at home, in the storm.
"After this match we are more happy, three points, but we have to continue and don't stop. Next week we are playing again and it will change the way we see the team. Today is a good day, let's enjoy it for 10 minutes and then think about the next game."
Amorim can also reach the semi-finals of the Carabao Cup this week.
His side visit Tottenham in the last eight of the competition on Thursday night.
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