Hostages of history
Three causes: a club that never replaced Ferguson's structure, a manager whose principles stopped adapting, and a media war neither side could win.
There are, I think, three key causes of United's decline under José Mourinho (and that it is indeed a decline is shown by a whole series of rather unbelievable statistics: 36 shots on goal, which is what the Red Devils allowed Liverpool; 29 goals conceded in the league, which puts them among the worst English clubs; a record-poor points haul, the like of which hasn't been seen since 1990).
The first, structural cause, which we wrote about a year ago, concerns the club itself. Alex Ferguson's departure represented an organisational, ideological, operational and value-based shock for which United didn't know how, or more precisely couldn't, prepare. By all accounts coming out of United, the gap behind their rivals is deep and dramatic, from scouting and player selection systems to a neglected academy and the absence of any long-term sporting strategy.