The Manchester United midfielder lined up in a different position against Crystal Palace at Old Trafford.
Mainoo was deployed as the Red Devils' centre forward in a false-nine role for Amorim's side.
Meanwhile, strikers Rasmus Hojlund and Joshua Zirkzee were named on the bench.
Manuel Ugarte and Bruno Fernandes started in midfield behind Mainoo, with Amad Diallo and Alejandro Garnacho out wide.
The decision to play Mainoo up front stunned United fans, although they were also quick to notice the 19-year-old's impact in his new role.
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Reacting on social media, one supporter wrote: "Mainoo's been playing striker for all of five minutes and he's already showed more box intelligence and movement than Hojlund and Zirkzee have showed in months."
Another added: "The way Mainoo is holding the ball is different to Hojlund."
A third fan also commented: "Mainoo uses his body better than all our forwards in the box."
Hojlund and Zirkzee have both struggled to establish themselves so far this term.
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Between them, the pair have only scored 11 goals across all competitions.
Diallo and Fernandes, meanwhile, are United's top scorers with nine goals each already.
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Sunday's clash with Palace was Mainoo's 15th appearance under Amorim.
Since the Portuguese's arrival in November, the United academy graduate had almost exclusively featured in his usual midfield role.
However, Mainoo did score and assist as an attacking midfielder against FCSB in the Europa League on Thursday.