I think the developers mixed up a bit in that field.
There are two separate fields one is stamina and one if game shape, most of the times when a player is injured he can still keep his stamina, for example if a player break a finger he can't play, nevertheless he can run and keep his stamina, what he should loose is game shape.
Also as the rules says, there is nothing between stamina and minutes played, to run around the building players do not need game time.
So, stamina should drop during time if you don't train it, but has nothing to do with game time.
Completely agree, particularly with the part in bold.
One other thing that really bothers me about this is that the stamina drops may effect BuzzerBeater's ability to retain new managers. As a newbie, you are given a team with 18 players, normally on the day of the training update, and none of your players has played a single minute. The training update takes place, and all of your players drop in stamina. During the next 7 days, you have to figure out which players are worth keeping, and try to give each of them 24 or more minutes (I've been playing for better than a season, and I still can't figure out how to get 13 players 24 min/week). Those that don't get 24 minutes will suffer another stamina drop. Reading the rules tells you there is no relation between stamina and game time. You train stamina the next week, and you still have players dropping in stamina. Will you keep playing? How many n00bs do you think will stick with it?
If the developers really think that there needs to be some challenge to maintaining stamina, there are better solutions. First, if stamina must drop as they have currently implemented, there should be a % share of each week's training devoted to stamina, in addition to whatever training is the focus the team. Second, if that option is not available, then CharlesThe2nd has the bast solution:
So, stamina should drop during time if you don't train it, but has nothing to do with game time.
And this should be a very gradual drop, which could be tweaked to be less gradual for a player who doesn't play much at all for weeks on end.