Hi I would suggest giving a warning on the formation page when an illegal formation is set.

I have a friend player on here that has an illegal formation set( just 2 defenders) He/she was logging in daily but I've noticed he/she has started logging in less, probably because he/she is losing almost all his/her games. He/she is just a friend on the game so I've no way of informing him/her. I imagine he/she is not the only player in that situation.