Unfortunately there is no way to get them back when you release them. Once they are gone they are gone, I sold an 8 and a half star player I won on from an event:
I am sorry this happened to you, you need to be a bit more careful in future though, I definitely agree that there should be an ‘are you sure you want to sell this player?’ or if you bid for a terrible player on transfers by accident there should be a withdraw button on a player so you can remove your bid if you want.
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