There's been many things that have went wrong.
His transfer dealings have been quite poor. I don't feel he ever got on with Di Maria. I appreciate AdM didn't really want to be here but if he was given the freedom and constantly picked as he should have been then who knows, maybe he would of stayed and flourished.
The likes of Welbeck, Evans, Nani and Rafael could definitely add something to this team, and should have been kept. The reluctance to sign a CB has puzzled a lot of people and doesn't look a wise move. Blind has done okay but if you analyse his performances it's clear as day that he's not a CB and it's probably due to him being an average CM that he's playing there. That may seem harsh but if we want to progress and be the best again, Blind should not be a regular starter in either position. Under Fergie, he would have been utilised very well, coming in at CM in some games, covering at LB and filling in at CB when needed.
In addition, even when winning games back in August, a big alarming point was that Carrick/Rooney had to speak to him to cheer the dressing room up, that shouldn't be happening when a team is winning games especially at the start of the season when there's less pressure.
Something that surprised me when watching our pre-season training was Van Gaal instructing Rooney on how to finish, surely other managers don't do this unless they have been the very elite i.e. Simeone and Guardiola and even then I highly doubt they would be instructing their strikers (top top strikers who have been round the block) on how to finish.
I understand attacking coaches using phases of play in how to approach the game and how to maximise their effectiveness in the box, or even general tips for younger strikers but Rooney has been one of the best players over the last 10 years and I'm sure he knows by now how to finish. I can't imagine the players warming to them type of instructions. Tactically and structurally I understand managers giving orders but based on technique, I'm not sure it's wise especially with experienced players.
For me, LvG can see a good game and knows how he wants to play the game but he hasn't adapted to the Premiership, it really is a different beast compared to other leagues, as has been mentioned all the successful teams were about power and counter-attacking football rather than tiki-taka football. Arsenal with Adams, Vieira, Henry and two speedy wingers, United have always been known for their wing play and counter attacking football while Leicester are showing just how effective it is this season.