Is this the cause of Chelsea's self-destructive defending?

How Chelsea's backline has transformed from the Premier League's most resilient to one of its most porous over the course of the summer is nothing short of mindboggling.

Indeed, Chelsea finished last term with the fewest goals conceded, the most clean sheets, the second-fewest defensive errors leading to a goal and the sixth-fewest shots conceded per match throughout the division. This season, they've conceded the second most goals, the sixth-most shots per match, committed the third-most errors leading to a goal and are still waiting upon their first clean sheet after four Premier League fixtures.

The two sets of statistics are almost incomparable. In fact, they're almost a reverse image of each other; as if an off-season mirror is separating the title-winning Chelsea of last year with this season's dystopian manifestation of the west Londoners. So what has caused such a seismic shift in Chelsea's defensive fortunes?

There's been a great debate over the need for new recruits in the last few weeks - particularly Everton's John Stones - but it's not as if the Blues' tribulations were foretold by the punditry alumni; when BBC Sport asked 30 pundits their top four predictions for the season last month, only eight dared to bet against Chelsea retaining the title and just one - Alistair Mann - put the Blues outside of the top two. The assumption being that Chelsea's 2015/16 campaign would be yet another built upon watertight defensive organisation and positional discipline - the monolithic cornerstones of the Jose Mourinho philosophy.

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The Portuguese himself has made no secret of where he thinks the problem lies - individual performances. In fairness, the Special One even criticised his own performance last month but by no means with the same conviction as he has those of Branislav Ivanovic, John Terry, Cesc Fabregas and Nemanja Matic.

Even if he hasn't named them outright, he's certainly shamed them. Terry's half-time substitution against Manchester City, the first time Mourinho's ever hauled off his skipper in 177 starts, said far more than words ever could. Likewise, it's not difficult to read between the lines of the Serbian international's harrowing slump in form and Mourinho declaring; "I blame myself for not changing one of them," after Saturday's defeat to Crystal Palace.

To an extent, he has a point. For example, no Premier League defender has been dribbled past more than Ivanovic this season, eight times in four outings, and Mourinho's concerns over Terry's displays allegedly root in pre-season. Similarly, Fabregas has struggled to rekindle the honeymoon form of his first few months with Chelsea for the vast majority of 2015.

Yet I see Chelsea's defensive frailties as something more systematic than simply a few rugged veterans falling into decline. After all, Terry and Ivanovic were considered to be the best defenders in the Premier League just a matter of months ago, a widespread consensus epitomised by their inclusions in the PFA Team of the Year, so there must be a more logical argument to their dramatic dip in form.

Looking at Chelsea's back six - and particularly the aforementioned Ivanovic, Terry, Matic and Fabregas - there's an obvious weakness that combines them all; a lack of natural pace. Gary Cahill is more agile than most central defenders whilst left-back Cesar Azpilicueta is by no means slow off the mark, but neither possess the energy or acceleration to dependably accommodate for those surrounding them.

This theory is nothing new - it's the same back six that started the 2014/15 campaign - but the difference last season was Chelsea's shape, organisation and balance. Ivanovic's limits were compensated by the duracell-charged Willian in front of him, who has moved to the central attacking role since the 2-2 draw with Swansea City, whilst Azpilicueta and Matic protected Terry, allowing him to resemble a world-class defender in the situations he's still a world-class defender in; particularly at the near post and in the air.

That practice, however, relied upon everybody performing their job perfectly. Willian - now Pedro - anticipating when to cover for Ivanovic, Matic and Azpilicueta never leaving Terry a yard too exposed. But as much as Mourinho would like to think so, players can't perform their jobs perfectly every time.

Likewise, it's abundantly clear opponents have identified this specific weakness; it seemed Manchester City added Raheem Sterling to their left-hand side at the cost of £49million almost exclusively to run Ivanovic into the ground during the 3-0 defeat at the Etihad, whilst Palace's powerful, pacey and direct wide-men need no invitation to exploit a backline's lack of speed. The same can be said for Swansea City's Jefferson Montero, who had an absolute field day against Ivanovic during the season's opener.

Curiously, perfect balance has always been synonymous with Jose Mourinho's teams. They attack only at the velocity that won't expose them defensively, whilst every individual strength and weakness is balanced throughout the starting Xi. Suddenly, however, Chelsea have an imbalance - and an intrinsically detrimental one at that.

There may be no John Stones at Stamford Bridge this season but the Blues' back six needs an injection of pace from somewhere. Perhaps it's time £18million signing Abdul Rahman Baba, a full-back famed for his prodigious talent and speed, made his Premier League debut, likely at the expense of Ivanovic.

Similarly, in the absence of the Everton defender, one can envisage Kurt Zouma being given a recurring role in Chelsea's backline this season.