United won 2-1 against favourites Manchester City.
Mainoo said: "(It means) absolutely everything. It's been a tough season with ups and downs. This is the only thing we've had to look forward to. Last year I was in the stands.
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"We knew we had to come together. The preparation for this game has been amazing. We've shown we can compete and win games."
Garnacho also beamed: "Incredible. Nobody believed in us. But we're a team, all together. We fought, the game of our lives.
"Incredible (fans). The fans are always there when we win, or lose, home or away."
On Mainoo, he added: "Incredible. The best player in the squad and he's the youngest. We're also good friends. I'm so happy for him."