The same I say everyday for my experience, if, a player have good statistics, keep this player in your team, it doesn't matter if he have 4 stars and all team have 7, if he, is programmed to have good statistics, for example.. when he have a level in the Attack habilities of 5*, he will work in this sense, and will be good always.
So, try to know your team, try to see if like in my case there is a player that in corner works well in the 70-80% of cases. And if you have a player for 2nd half, a ST or a MC that scores for you playing only 35 minutes, try to see that, and take advantadge of that.
My training is based in the Fifa experience, I always buy in this game the faster and resistant players, so, thats why I train first this side.
Other things to see are the hidden habilities... but, is what I say, I know my team, and you have to know your team bro'