The Argentine starlet was a constant thorn in the paw of the Turkish giants and opened the scoring with a left-footed effort just 11 minutes into Manchester United's clash in Turkey.
The Galatasaray fans certainly made their presence felt as they whistled Man United every time they touched the ball, leading Garnacho to goad the supporters after finding the net.
As well as doing Cristiano Ronaldo's 'calma calma!' celebration to quieten the fans, he then cupped his ear, much to the annoyance of those in the stands.
Galatasaray's goalkeeper, Fernando Muslera, was also clearly unhappy as he sprinted over to Garnacho mid-celebration.
Spotting the 37-year-old shot-stopper running over, Fernandes showed his protective side by intervening and blocking him from getting to Garnacho and keeping the two separated as the winger made his way back to his own half for the restart.
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The 19-year-old's celebration has clearly caused quite a stir, with Fernandes then being spoken to by the referee as he tried to calm the increasingly hostile atmosphere.
But that goal was just the start of the drama as Fernandes netted a stunner to make it 2-0 to the Red Devils.
Hakim Ziyech scored a free-kick to claw one back, before Scott McTominay made it 3-1 to put Man United in a comfortable position on 55 minutes.
Ziyech then scored a second free-kick which squirmed through Red Devils goalkeeper Andre Onana before Muhammed Kerem Akturkoglu drew the game level on 71 minutes to leave Man United's hopes of progressing looking rather slim.
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Erik ten Hag's side will now need to beat Bayern Munich in their final group stage clash and hope that Copenhagen and Galatasaray draw if they're to make it through to the next stage.