I know illegal formation affect possession. And also put players out of position. But i have been playing this game for the last year and i can tell you counter formations are unnecesary. Choose your better players, put them on their positions and of course, use legal formation (you know: three defenders, one striker, flanks...). And then you wait dices are with you. These stories that say if oponent use amc, then you must use a dmc...bla bla bla bla. Nothing real. The only important thing important with formation is use legal formations. That i can tell by my experience.