At least you beat them mate? Its Man Utd end of the end you cant go beating them 3/4 - 0 easily!
Look at the way that Man utd play, their team have alot of pace, so tell your team to press their players on all areas of the pitch, in doing so your not giving them any room to play their normal game. Also play a 4-1-4-1. You should easily control the Midfield game. Let me know how you get on.