Five reasons why it has all gone wrong for Manchester United this season
Lack of leadership: The summer departures of Nemanja Vidic, Rio Ferdinand and Patrice Evra saw United lose 1,096 games of top-level experience from their back four and, while nobody ever suggested it would be easy to overcome the loss of three such senior figures, the reality has been far worse than even the gloomiest predictions.
All three had shown signs of age and wear-and-tear last season, but allowing each of the trio to leave now appears to have been reckless.
United's remaining defenders are young and lacking in experience. Tyler Blackett needs to be guided through games, but so do Chris Smalling, Phil Jones and Jonny Evans, who often relied on the presence of Vidic, Ferdinand and Evra to cajole them along.
Confusion reigns: Are United operating a three-man defence or have they now reverted to four at the back? Louis van Gaal continues to evade questions over his preferred system for United, so supporters are as confused as the players on the pitch.
Van Gaal, who has typically played 4-3-3 throughout his career, has spoken repeatedly about the need to 'train the brains' of his United players, but for defenders schooled in 4-4-2, perhaps it is too mentally taxing to impose a 3-5-2 system on them.
No defensive discipline: The secret of any back line, whether it be three or four defenders, is to maintain a straight line when pushing up.
United, however, appear to have been operating with a zig-zag approach when attempting to maintain a defensive line, with the second-half collapse at Leicester a direct consequence of the lack of defensive discipline among Van Gaal's defenders.
There is no organiser on the pitch and while injuries have not helped Van Gaal in this area, had Evans, Jones, Smalling and Rafael been fit throughout pre-season, perhaps an understanding could have developed that would have avoided the problems exposed at Leicester.
But each of that quartet are often injured, so Van Gaal cannot rely on their fitness in the long-term.
Transfer market failings: Former Liverpool defender Jamie Carragher accused United of 'mismanagement' by failing to sign a top-class defender this summer, but almost £50m was invested in Luke Shaw and Marcos Rojo but little effort made to replace Ferdinand or Vidic .
Manchester City had a clear run to Porto's Eliaquim Mangala, who made such an impression on his debut against Chelsea on Sunday
Imbalanced team: Van Gaal has complained of inheriting an 'imbalanced squad' from David Moyes, one with more No 10s than central midfielders and a needless stockpile of wingers.
While that has been addressed during the transfer window, Van Gaal seems to have moved the problem around his team rather than solve it and it's no surprise that the defence is struggling with so many attackers in the team.
Only Daley Blind, a defensive midfielder, offers the back four any protection, while the likes of Ander Herrera, Angel di Maria and Radamel Falcao offer little or nothing in terms of defensive contributions.