To put it in short, Man United's squad doesn't seem to have the qualities necessary for the current system.
Louis van Gaal does not really seem to care what a player's strengths or weaknesses are: he has a foolproof system and you have to adapt to it. What's good about this is that almost anyone can play in any position and the result is going to be the same (as we've seen in this season). You could probably have the Norwich squad playing like this and they'd be in 5th or 6th place.
Our players don't seem to grasp the concept of positional play, which is very demanding and requires a huge deal of concentration, especially if you're new to it. Rooney's shocking form under van Gaal is primarily down to this. Good ol' Wayne almost always makes the wrong decision: he stands still when he should be dropping into midfield, he goes into a DM position when he should be making a diagonal run, he runs vertically when he should be running diagonally, he runs in a straight line when he should be fainting a bit, etc.
And then there's the issue of the speed at which they are playing. In modern football, if you take 2 seconds to make a decision, the opposite team will be ready for whatever you chuck at it. And United are waaay to slow at making decisions. In the match against Southampton, I don't think Lingard ever received a pass by just stopping the ball with his feet. He either bounced it off and had to run for it, or tried to bounce it up to then control it with his chest. Why on Earth would you do that if you have 2 rivals running at you? There's no need to take more time than the time you need, and yet you risk losing the ball or delay the game, which favours your opponent. Same thing with Rooney, when was the last time he went through 3 plays without making a mistake? Passing the ball directly to a defender, being unable to control a simple pass, shooting instead of giving a simple through ball that everyone can see. Either many of these players aren't mentally fit to play this type of football, or Louis van Gaal isn't fit to coach it properly.
If you go position by position, Man United probably has the 2nd or 3rd best squad in England. But in football, the team is what makes a difference, and United are below where they should be.
And Louis van Gaal isn't making the team any favours by going on analysing the game in his press conferences. He has his principles, but I'm sure Fergie would've centered the attention on Darmian's injury, or in Southampton's violence (regardless of this being an exaggeration) or in the good signs shown by Jackson. Don't stop and go over and over how rubbish we were and how rubbish we've been for the last 3 or 4 months.
We've always had what you could call "low quality players" in our squad. Shit, even Gary Neville is praised way above what he really was. And under Fergie we had O'Shea and Brown, who were extremely limited in their abilities, and yet United won a fucking Champions League with them. Same with Anderson, Cleverley, Young, Park, and so many others. Our backup goalkeepers have always been just a bit short of terrible (Romero is by far the best backup goalkeeper we've had in what... 20 years?), and yet United won league after league after cup after league.
With the current squad, I'd say United shouldn't be finishing the season below 3rd in the league. There is no way United has the fourth best squad in England. Anything below 3rd is the backroom staff's fault, even with 5 injured full-backs. United's current situation is not something that Valencia would cure.