I just said in another thread I think there's a "behind the scenes" bell curve for player performance. Some players have legendary potential, and will play well from day 1 until retirement; others will be stars for a few peak seasons; others play well enough, but fail under pressure (my GK who can go 5 saves, man of the match then down to a 5 rate in league, then right back to form); and others who should just be avoided and can't be developed.
Just like real-life, actually...
You forgot the players who play awful under you, but become superstars once transferred out, and vice-versa.
Any formation or tactics advice given is based purely on experience with my teams...