Npw what yopu do is a friendly vs a 1* team, to oversize the internal programming. Once you sack old players with key roles assigned, the options to get a good role assigned in a new player increase.
This allow you to discard easy, because discard players that don't convince you is part of the job, or, for example, if you see am aggressive role assigned to a player, you keep him as reserve so when you go to the market for a different option, you're preventing the assignation of that role.
Think that this have a margin, each role basically covers needed animations, so too depend on the dominance power assigned, but you can count that mainly in a obvious defined role you can have between 2-5 important players (those who assist, score, aggressive, in defensive animations etc and the) Its a bit like a imprecise formula but by testing you can spot your team combination.