It is not just one problem, Chelsea got to the Champions League final with Avram Grant, we won title after title with a really outdated club structure, Barcelona spend half a season without a manager and won the league.
When a club is being well run and has quality players and staff for the most part it is very possible to paper over certain cracks. We have problems, plural.
Edit: I don't expect anyone to read this but it doesn't half feel good to rant sometimes.
Structure of the Club - Sir Alex Ferguson was the greatest manager to ever live and had a quarter of a centuries worth of know how about the inner workings of the club. He acted as the head coach, the director of football, human resources, was involved in marketing, brand ambassador, essentially acted as a line manager for the managers of the youth team etc.
There wasn't a manager alive who I think could have come into a club the size of Man Utd and taken over that role. I think we realised that fairly quickly and Ed Woodward has attempted to take on some of that burden but he is unfortunately not qualified for that role despite being brilliant in other areas. The result has been hiring managers based on their reputation and blind hope rather than a long term plan that includes a style of play or an aim for team building. As an example we apparently went for Klopp before going for Van Gaal and there could not be two more different managers. We need a Director of Sport who will not just concern himself with the short term but implement a long term goal for what type of football we want to play, how we handle squad building including both new signings and promoting youth players, what the expectations should be of the manager and more. Once this is in place then everything from the managerial and coaching hiring, player recruitment, youth team setup, directive the scouts receive should all be targeted towards this identity. I also think there needs to be more pressure put on our scouts and there should be a new PR team put in place (how do we not have a youtube to rival Man City?) and player relations should not all be handled by the head coach.
Player Recruitment - David Moyes, Van Gaal and Mourinho are three very different coaches, that allied to the fact that we don't really have that identity in the way that a club like Barcelona do means that we have this weird imbalanced mesh of players. I remember reading this article once called "players that you can't believe played together" it feels like that with most combinations of our players. Our desperation for a short term solution has only served to make matters worse, we were crying out for a right winger last season and we signed Alexis Sanchez instead. We were all guilty of getting over excited and thinking that we should just get the deal done "announce Sanchez!" and then worry about it later but that has never been how good teams are built.
We have also developed a habit of signing players at terrible times. Falcao just after he came back from injury, Sanchez half way through his worst ever season, Bastian when he was clearly on the decline. Our record outside of this has been mostly average with a couple of bright sparks. If you look at the way City have spent a similar amount since Sir Alex retired there is a huge contrast.
Additionally we have let players who were just as good or better than what we have go for very little return. Evans, Rafael, Michael Keane, Zaha, Kagawa and Blind were all sold for less than we spent on Lindlelof and Darmian. For me they would all still do just as good a job or better than the players in their positions currently, not world class players but players who we already had at our disposal and let go for peanuts to replace them with more expensive versions who performed worse. I am far from opposed to cutting dead wood but considering what we have gotten in return it just seems incredibly wasteful.
Management - Moyes won't get mentioned, the choice to hire him falls firmly in the "not having a plan" camp. Van Gaal and Mourinho are excellent managers, this has been proven throughout their careers. However we need someone who is going to lay the foundations for a team that is going to develop over a number of years. Van Gaal can do that, if that is what he was hired for then the board needed to keep faith in him and let him continue with full backing. I think may have gotten us playing really well with enough time but he was ready to retire which poses the question why would you hire someone to start a long term project when they are going to retire in a couple of years? Didn't work out. They then completely change tact and go for a short term super manager, unfortunately that short term success hasn't come. Now we have a failing short term solution attempting to fix a long term problem. Jose should not get the blame for this but he is so clearly not the right man at this point.
Style of Play - We don't run. We covered the lowest amount of ground in the league last season, our players are completely static unless they are in possession. Due to this when a member of our team has the ball their teammates are all being marked and they have to try and thread a pass through the eye of a needle. This makes players like Paul Pogba, Matic, Sanchez and Mata look like bad passers of the ball, they aren't. When the opposing team are on the ball they are rarely pressed and when they are it is not nearly aggressive enough, this allows them to advance up the field too easily and often results in last ditch challenges.
We are statistically the slowest team in buildup play in the league. In boxing they say that the punches that you don't see are always the ones that knock you out, this is because when you see shots coming you can angle your head in a way that it doesn't land cleanly and clench your jaw. This is very similar with football, when you attack very slowly and predictably defenders can always be in the right place. There are so many smaller teams who play against us and end up being praised for great defensive performances where they "don't give us an inch" my god to we make it easy for them.
Momentum - Just a small point but I think this gets underrated in football. Sometimes you just get a feeling that a team is in the ascendancy, confidence starts flowing through them and things just have a magical way of falling kindly. We are the opposite of this right now and our players confidence is shot to pieces. Sir Alex was better than anyone at creating this, we would lose a game and he would put together a run of 10 wins and suddenly we were flowing.