The best teams all have a defensive and offensive system that they follow. United seem to completely lack an attacking system. Lets be honest here though, we aren't exactly a rock at the back either.
I think part of our issue is players underperforming. When you are winning, smashing the opposition and generally dominating most of your games you have a lot of confidence and the whole team is relaxed. We don't have that so when we attack we are a bit staccato, misplace passes and generally look like we are very disjointed. Mourinho isn't a manager to nurture his players so that just gets worse over time. I can understand our managers frustrations over the past few years because you can be the best team and make the best chances all game but if your players are making simple mistakes then you will have problems. When we counter attack it usually breaks down through a misplaced pass or a bad run or simply someone being greedy.
We have a manager who wants to coach his team to be a strong defensive unit, break on the counter and punish teams. What we have is a team that is a bit suspect at the back and a forward line who miss too many chances and ruins far too many counter attacks. If teams were getting 3 or 4 goals put past them when they tried to attack us they might think twice about it. As it is though even the minnows of the league fancy their chances. They don't fear our attack because we are so wasteful. Its hard to overstate how beneficial it is to have teams petrified about your attack. City and Liverpool have this and it has so many benefits. Teams don't commit so many men forward, your defence is tested less so its generally more confident, your defence isn't conceding as many goals, your whole team can join in the attack, your attackers are scoring more goals. Literally everything is easier when teams worry about your attacking players.