It won't be illegal formation. You might get a slight possession penalty for out-of-position players. I once got my GK injured and didn't have a sub. they rarely get injured. Only once in 6 seasons did my GK get injured. I made my DR play as the GK. It was a matter of 3 days and he did just fine. Won the 3 games as far as I remember. He conceded about 1 goal and got an average rating of 6.5. So it was manageable.