Even though your players will initially be placed in a default formation when you start playing, you will be able to change your starting lineup, formation and tactics depending on your preferred style of play.
If you place a player on the part of the pitch not related to his role, he won’t be able to perform as best as possible, with his OVR being lower compared to playing in his position.
Go to the Squad screen and tap on the “Manage” button (located in the bottom left corner) to change your starting eleven players, or just adapt formation and tactics.
You can choose one out of predefined formations, so make sure to find the one that suits your Squad the best.