After noticing loads of factors in this game and reading carefully all posts in forum I can say I noticed few important things.

First of all I started observating the form of the players in each game.
I have some DML that is performing from 6 to 5 in each game and unfortunatly he is my best player on this position. I had to put a 26 year old with 3 quality less that is performing from 7 to 8 on each game. After benching my original player more than 3 days I can see him again performing a 7 on each game.

There is also another important factor for the performance that is the mentality of the team. I saw form dropping or raising on the change of mentality from a game to another. So my conclusion is that every player is different and it depends on how this player is formed to play.

I try to play the same system in every game and just watch my player's activity during the game. Unfortunatly I must say that if a player is performing bad for a serie of games the only solution is to replace him, even if it's your top player.

So the point is , like in real football, to have only the best 11 starting. That doesn't mean the best in quality but in form. I saw that the performance of the team is way better that way. And try not to change those players position.

My latest experiment was with a player playing as DML and DMC. I compared him to another DC/DMC and saw that the DC/DMC was much heavier and when I switched them having my original DMC play at the other position he was assigned to play (as lighter player) as DML his form went from a regular 7 to 8-9 in each game. So I guess only by experimenting and observating mainly the form of your players in each position you can really have them performing to the MAX.