It happens to all, it seems that GK and MCs always receive the worst ratings.
The only thing you can try is changing sides, Put the one in the left playing in the right and vice versa, sometimes helps.

Besides that just ignore, i really do not care much about the ratings, If they score, make assists and so on, just play the game.
Last season i had a MC who scored 10 goals, is normal rating was 6, no matter If he scored or not.