Garnacho lasted 120 minutes and sprinted three-quarters of the pitch to lay on the final minute winner for Amad Diallo.
Younger brother Roberto, in the aftermath, revealed Garnacho was a doubt going into the game.
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He posted to social media: "Garnacho has trained two days this week due to a hamstring injury. Still, he is there running 120 minutes and giving the winning assist.
"I'm proud of you BIG BRO."