Stamina does play a key role in performance as well as minutes. I'm not sure if anyone knows the exact mechanics of it. Things like at what point in the game a player starts feeling the effects and stops playing up to his capability. I know that when I am training players that all of my players drop in Stamina because I only worry about training minutes. I try to train my youngsters to a capable playing level and then train Stamina for half a season or so. You will notice that you get more quality play, deeper in games, with higher stamina. This also rings true for the transfer list. If you have a guy that should sell for say... A million.. And he has a 1 or 2 in Stamina, you may get 800 or 850k for him. But on the other hand, if he has 8-9 in Stamina, you may get 1.2 Mil for him. It is a talent that people hate to train, but it has plenty of benefits. Exactly what the benefits are, I am not sure anyone but the game makers know. Anyway, sorry about my long rant!