What has gone wrong with Manchester United's defence? Jose Mourinho's problems have been long coming

United's defensive record was second to Manchester City's last season, but now they are the fifth-worst defenders in the league

"Leonardo Bonucci and Giorgio Chiellini, they could go to Harvard University to give classes about how to be a central defender," he marvelled. "Quality in football is everywhere. When I say quality I don't just say creative players. Chiellini and Bonucci, it is absolutely beautiful to see." Watch yourself Jose, you're drooling.

Who can blame Jose Mourinho, though, when Juventus arrive in town with two of the finest examples of old-fashioned defenders, when his equivalents are the suspect Chris Smalling and Victor Lindelof, and when Manchester United's own defensive record is now becoming quite alarming?

Paulo Dybala's decisive goal in Tuesday night's Champions League tie was the 19th conceded in 13 games by United this season. David de Gea has mustered just three clean sheets along the way, with just one coming in the Premier League.

United's defensive record was the top flight's fourth-worst when the final whistle sounded on Saturday's 2-2 draw at Stamford Bridge. It ended the day the fifth-worst, but only because Manchester City did their rivals a favour by battering Burnley.

Meanwhile, the underlying statistics suggest that United are giving up better quality chances than the other members of last season's top six. By any measure, Mourinho's defence simply is not operating at an acceptable level for a side that should have aspirations of winning the title.

More concerning than any statistic which may even itself out over the course of the next few months, though, is that there seem to be few ways to plug the gaps without adding to the current squad.

Some wondered why Mourinho prioritised a central defender in the summer, given that United could boast a defensive record second only to City last year. The reality is that the position has been in a state of flux since the respective declines and departures of Rio Ferdinand and Nemanja Vidic.

Smalling and Lindelof are currently the men in possession, but Mourinho has his doubts about them as well as Eric Bailly, Phil Jones and Marcos Rojo. His answer, at times, has been to throw midfielders Ander Herrera or Scott McTominay into a back three, though it is unclear how increasing the number of centre-half positions is supposed to help.

Jose Mourinho is jealous of Juventus' depth in defence (Getty Images)

The back three experiments have come and gone quickly, while the centre-half pairings have chopped and changed. Six different combinations have been used thus far. After the defeat to Tottenham Hotspur in August, Mourinho openly admitted that he does not know the make-up of his best defence.

The only certainty is that De Gea plays behind them, yet United can no longer be confident that one of the best goalkeepers in the world will help paper over their defensive cracks. The Spaniard looks human again after an outstanding campaign last year but a difficult World Cup.

Having prevented more probable goals than any other goalkeeper in the Premier League last season, according to expected goals models, his numbers have gently come back down to earth at the start of this season.

There is no shame in De Gea falling slightly from the heights he reached last year. Yet he was so excellent and so critical were some of his performances, that any slight drop-off this season would have consequences on results. The blame does not lie at his door, though.

Is there a better illustration of the depth to United's defensive issues than the fact that no goalkeeper in the club's history had won the club's player of the year award before De Gea's first in the 2013-14 season, Ferdinand and Vidic's last year at Old Trafford, but De Gea has won the award in three of the four seasons since?

The problem is a long-standing one of personnel and, on that, Mourinho has been vindicated. For all the criticism he is due, for all that he could be doing better - and while bearing in mind that he sanctioned the signings of Bailly and Lindelof - he was right to seek at least one commanding, unifying presence in the heart of his defence.

The club, however, believed there were no stand-out alternatives, leaving Mourinho to salivate over the likes of Chiellini and Bonnucci, and leaving us to wonder what difference a player of their ilk could make.