Help against 4-3-2-1

[July Fourth]

Sorry for the wrong title, don't know how to change it.

Is this formation legal?
5-1-1-1-2

I'd say no but he has won the last two matches..

Yes... It's legal. It's just a variation of 5-3-2 with the 3 midfielders lining up in a straight line of the middle of the field.