Whenever we want to expose data to Views, we need to define this data in a way Views can understand it. That is actually what EntityViewsData::getViewsData() does for content entities. However, since we are dealing with something custom, we can do so by implementing hook_views_data(). A lot can go into it, but we'll start things simple.
Let's implement this hook and simply describe our first table (that of the players) and only one field, namely, the player ID, to start with.