why for example a player that is injured and doesn't play any minutes always drops in stamina then?
I see this question wasn't answered yet and maybe others want to know to should you already found out:
What you tell was true in seison 2 and 3, but (I believe it was) in season 4 that they altered the stamina system to prevent players with 2 or more weeks of injury to drop drastically in stamina. Now every player has (on average) a steady small drop in stamina which makes it so that you need to train them once in a while, but they won't suffer from playing minutes anymore.
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