Opinion: Why Chelsea couldn't lodge a proper defence of their title

As Antonio Conte’s Chelsea prepare to face Manchester City on Sunday, we discuss the possible reasons as to why the Blues have not been able to offer a proper defence of their title.
The Blues are already 32 points behind the Citizens, who are leading the standings, and it is now more of the matter of when the Mancunian outfit will celebrate lifting their third Premier League crown.
Two reasons for the Blues’ failure to defend the title:
Departure of key players:
Whilst their rivals parted ways with out-of-favour players, Chelsea made a surprise decision to offload to Nemanja Matic to Manchester United last summer despite him playing an influential role in the Premier League triumph. Obviously, the decision has not favoured the Blues with his potential successor Tiemoue Bakayoko struggling to adapt to the pace of English game.
Likewise, the Blues have lacked the presence of a potent striker in their ranks following the departure of Diego Costa. Alvaro Morata made a quick impact following his summer arrival with six goals in as many appearances, but he has since struggled to make the most of his chances. The Arsenal game back in January was probably the best example where he should have had a hat-trick in a 2-2 draw. Instead he failed to find the scoresheet.
Antonio Conte’s formation: 
Over the course of the season, Antonio Conte has gone with three-at-the-back irrespective of the results on the table. The Italian has had huge success with the setup during his time with Juventus and the Italian national side, but his reluctance to change the system while trailing usually brings a frustration among the club’s faithful.
Chelsea had a much-better defensive setup with Gary Cahill and David Luiz playing alongside Cesar Azpilicueta, but rather surprisingly, the aforementioned pair have dropped their performances levels this term. With both players putting in an average showing, the manager has gone with the young Andreas Christensen and Antonio Rudiger at the back, and they have had a few jittery moments in terms of positioning, while adjusting to the rigours of playing across four fronts.