His formation is legal. But he was weak compared to me, and my formation counters his, that's why I had 80% possession. So because of that game, and the game I lost today (again, against an weaker opponent whom I had won against in his ground, but not by that much), I fell from the 1st place (and I was leading the league for much of the round) to 5th, while having the strongest squad in my league.