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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It means an automatic one-match ban, but for multiple yellows the suspension applies to the next domestic game. Therefore he will miss the Premier League game with Brentford on March 30, before returning the following midweek when they face Chelsea.
It also definitely means Amad will be available for the FA Cup semifinal against Coventry in April.