I think that the manager has a set formation in his locker, then he buys the correct players to suit his formation (like we should) then he will set up the players with some instructions the goodness (if thats a word) of the instructions will be dependent on how good the manager is, so a poor manager might make mistakes and a good one will give better instructions, hence why good managers get with good teams!.
Also if they had 1 way to play and a set amount of instructions(set by si at the start of the game), then after a few seasons when they get new players ect.. there tactics would become useless and they would start to do really badly, so hes always ajusting his tactics to suit his players, hence called AI