Suggestion is to introduce player morale as stat which influence performance of players.

Player Morale will change based on grades given in match, for example when got grade 7.00, it will not change Morale, grade 8.00 will increase Morale for 1, and grade 6.00 will decrease by 1. 

Additional building psychological center can be built which gives option to increase Morale for wanted player based on level of building.

Before you do that you need to modify the grades for players.

For example, if a defender has nothing to do in a game he receive in this moment grade 5.

Why he should decrease morale if the game has been won with a clean sheet and nothing to do on defensive side?

Agree, grades should be revised, even worse is for substitutes, they come in 90 minute, don't do anything and get grade 5