There are more rules besides this, same goes to Michael. And Michael, he did much more wrong than just playing with a stupid lineup, if he had played a legal formation, even if the opponent found his weak spots, I believe he would have won easily.
I would say you need at least 3 midfielders, that's all we can speculate here. It's worth noting that 5-2-3 is the only possible formation with less than 3 midfielders.