FK man and CK man are positions that the game uses to make you test and manage, so the game will select a player to be the best for these 2 jobs, and they will perform better than anyone for some weeks, as the game isn't so paranoid giving contributive roles, a key player is a key player for weeks till the team configuration changes of so, and thaths the same.

So yeah, if the GK assists/scores from CKs / FKs and does it better than anyone else, he is your man.