On the surface, it seems rather absurd - a backline boasting 974 games' worth of Premier League experience, including arguably the greatest academy product, John Terry, and the best value-for-money purchase, Branislav Ivanovic, in Chelsea's history.

Yet, for a team so highly regarded throughout the game as one of Europe's most formidable defensive outfits, the numbers don't quite add up - particularly, the Blues' average of conceding 0.9 goals per match in the Premier League, during a season in which many view the quality of the English top flight to have reached it's lowest point for over a decade.

To give some comparison, Southampton, who claim the healthiest goals conceded column and the joint-most clean sheets in the division, concede only 0.7 goals per match, despite their backline being assembled on a shoestring (around £14million), and the Chelsea side of Mourinho's first stint at Stamford Bridge conceded just under one goal every two matches in the Premier League. Likewise, last season, their rate was 0.7 goals per game.

During the current campaign, in contrast, the Blues have let in five at White Hart Lane, four on their own patch against League One side Bradford City, and conceded to Burnley, Hull City (twice), Aston Villa and QPR - four of the Premiership's seven lowest-scoring sides.

Is Chelsea's defence, widely viewed as the best in England, actually rather overrated?

To some, such a hypothesis verges upon sacrilege. Even when operating with knees ground to dust at the veteran age of 34, John Terry is still undisputedly one of the best - if not the best - near post defenders in world football. Similarly, there are few right-backs offering the physicality of a centre-half whilst conjuring four goals and four assists in 29 appearances at the other end, including a particularly special left-footed piledriving volley against Aston Villa, quite like Branislav Ivanovic.

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Cesar Azpilicueta has transformed himself from an adventurous right-back into the Premier League's most formidable defensive left-back since Jose Mourinho's arrival in summer 2013, and although Gary Cahill has emerged as somewhat of a weak link in recent months - his schooling from Harry Kane during a 5-3 defeat to Tottenham, his passive role in Bradford's 4-2 FA Cup comeback, his flat-footed attempt to stop an Edinson Cavani header in Paris - the 29 year-old is still England's first choice at the heart of defence for good reason.

That being said, Mourinho's always made the game as simple as possible for his back fours. We've seen a more expansive style from the west Londoners at times, with a higher line and Ivanovic particualrly bombing forward to compensate for Willian's limited width - perhaps shedding some light on why the Stamford Bridge outfit are conceding more regularly than last season. Yet, the Blues' ability to defend as one single, well-organised, cohesive unit remains unquestionable, so when compared to the likes of Arsenal, Manchester City or Manchester United, Chelsea's backline rarely finds itself exposed at all.

Likewise, the Blues defence is sandwiched between two of the best in the business at their respective trades - the 6 foot 6 Thibaut Courtois, now only comparable to Manuel Neuer in terms of stature and ability amongst Europe's goalkeeping elite, and the equally monolithic Nemanja Matic, sweeping in front of the back four to register the third-most tackles per match, 3.6, of any Premier League player this season.

With that kind of protection and reputation as individuals, you'd expect Chelsea's back four to cope with the slightly increased demand this season as Mourinho's allowed his side more freedom going forward. As their campaign's dragged on, however, Chelsea's defensive weaknesses have only become more noticeable; firstly, the aforementioned decline of Gary Cahill, seemingly trapped in a purgatorial cycle of continuously backing off in one-on-one situations, and secondly, an abundant lack of pace throughout the Blues' back six, starting at Courtois and ending at central midfielder Cesc Fabregas, with perhaps the exception of Azpilicueta.

Don't get me wrong, Chelsea's defence, the experience, quality and organisation they've shown, has stopped wins from slumping into draws and draws turning into defeats on plenty of occasions - especially against top sides, where the Blues have surprisingly struggled this term. But once again, it's been as much a collective effort, epitomised best by the dogged servitude of winger Willian, as it has a departmental one, and when Chelsea's back four have lacked support from midfield, especially on the counter-attack, they've often struggled to cope.

That's not to suggest Azpilicueta, Cahill, Ivanovic and Terry are in any way undeserving of a Premier League title, but perhaps this is the ultimate litmus test - would any actually make your world Xi at this moment in time? Personally, I view the defenses of Barcelona, Bayern Munich, Juventus, PSG and Real Madrid as superior in quality individually, even if some struggle to match Chelsea's level of organised unison.

Almost certain to enter the summer window as reigning Premier League champions, it's something Jose Mourinho must long-consider. No side has retained the English crown since 2009, so improvement during the offseason will be essential to Chelsea's title defence. With three of the back four now into their veteran years, perhaps it's time for a little reshuffle.

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