As was already mentioned, Manchester United is a very versatile team. Generally, it is offense-minded, so using offensive mentality is a good option. Especially with such potent Center Backs as Vidic and Ferdinand. But frankly – there is not much need for this, as a balanced game favors MU against almost any opponent. Only those teams that have 5-man midfields can disrupt Man United's control of midfield. Defensive mentality is also an option – but not for long periods of time, since you don't want your opponent to control the game. Moreover, the midfield (Carrick, Scholes, Anderson) is generally offensive minded and will not help your defense too much.