I play this evening an opponent playing 4-2DMC-AML-AMR-ST-ST. The possession stats of the team looked normal in the previous league matches (worst possession ratio was 43-57), so I don't think that not having a ML, MC, MR (unconfirmed case) leads to an illegal formation.
The formation looks like a variation of the hexagon, so I will try a 4-4-2 against it.