The Ivory Coast youngster scored the winner for United in a 4-3 FA Cup quarter final success against Liverpool.
Diallo's 120th minute winner saw him take off his shirt in the celebrations, resulting in a second booking and a red.
This moment has to be enjoyed and I think it's one of the rules football has to change," Fernandes said post-game.
Because you should be able to celebrate the goal, obviously with respect to other clubs, to enjoy your moment.
The thing with Amad, he got his reward because he's doing the right things. When you do the right things, you get a chance, and what is yours comes to you.
He deserves it. Unfortunately, he got sent off, but it's part of the moment and part of being young."