The opposing formation isn't illegal, it just have missing posts due to contract expiring.

The game will auto sub players in when that happens, it remember the missing posts and automatically replace a player for the starting lineup like when an injured players is there.

As there are 6 players at the front like ordinary 442, it could be countered by defensive mentality and long passes.
But don't use 5 defenders with defensive mentality, as you have no AMLAMR you may try 433 or 4312