Damn, just observing slowly this, their defenders are not blocking the opposing players, but instead are watching their friends who are competing with the opposing attackers, without realizing that several other opposing players are already in front of them or even beside them very close to the goal. This is one of the reasons why their defense is so weak and they often concede goals. Some of them do not understand their positions and duties especially in critical moments like this. And unfortunately, moments like this often happen at Man United.

For some reason, this illustration really represents the condition of their defense.  

Source: Manchester United's defense against Tottenham Hotspur