I assume you are very new to the game and this is your second month (season).
We have all been there and hit with this thing. But usually I don't really give much thought about cups because it is jst the luck of the draw.
Having said that, like tarcanes92 said, end of the season, identify players who will be out of contract and drop them to the bench in case you don't log in before the cup match starts. Else jst renew the contracts early but you'll lose a year. I used to do that coz didnt want the hassle of rushing into the new season.