#3 His neglect of the midfield
The hallmark of most great teams is the functionality of its midfield - just ask Barcelona. And while many teams get by without an adequate midfield, more often than not, a team without a great midfield is set up for failure.
It is in this regard that Sir Alex Ferguson failed in preparing United for the future, as he totally neglected the middle of the park, failing to sign a central midfielder for the last six years of his tenure.
The best example of the dearth of midfielders at the club during this period would be that Paul Pogba left the club as a then inspiring youngster, having been on the bench when United needed a midfielder, with Fergie choosing to send on Rafael who is a full-back instead (this would not have happened had the club been stocked with midfielders).
Perhaps a better example would be the rather ludicrous return from retirement midway through the season for Paul Scholes which highlighted the lack of quality in midfield at the club that time.
Manchester United re-signed Paul Pogba for a then world record fee in 2016, and while he might be a quality player, United have been lacking in midfield dominance, as was the case during the days of Paul Scholes and Roy Keane, and SAF had a role to play in United's lack of midfield identity.