Diallo was sent off for his celebration after scoring the final goal in United's 4-3 quarterfinal win against Liverpool.
Amad was shown a second yellow card, having been booked four minutes earlier for tripping Reds captain Virgil van Dijk.
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Halsey wrote for The Sun: "The Manchester United winger's actions were silly because referee John Brooks had NO option but to show a red card for his celebration.
"This law came in because of players making political statements.
"It wasn't the case here, so I'd like to see the law changed as fans want goals and emotional celebrations.
"If Brooks showed common sense, he'd have been rapped by his bosses for failing to apply the law correctly."