Im not even sure the overall quality of a player really counts.
Ive had quite a few players over the seasons who have been 6* star players but have been awful.
For example I've got a AMR just now who has just become a 6* star player and ive tried everything with him, I always play him in his right position, he always has high morale and condition, I gave him a run of 20 games in the team, ive then tried and rested him for a few games then gave him another run of games in the team but no matter what I try he always gets a rating of 4 or 5 with the 6 in matches.
I had the same thing a few seasons ago, I bought 2 scout MC and their average rating after 28 games were 5.3 and 5.6.