Chelsea are currently enjoying a six-point lead at the Premier League's summit, as many pundits tip them to walk away with the English title undefeated this year.

Yet, ascoreless draw with habitual bogey team Sunderland on Saturday suggested things might not be quite so rosy at Stamford Bridge (or should that be bluey?) as the Blues' pole position status might initially suggest.

Indeed, a few weeks ago it seemed their title bid was all-but confirmed. Now however, doubts are beginning to creep in.

So with that in mind, here's FOUR reasons Jose Mourinho's boys could eventually let the English crown slip from their grasp come May-time.

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Overreliance on key players

Whilst the rest of the Premier League are hell-bent on utilising the far corners of their expensively-assembled squads, to paraphrase Mirror Football’s Andy Dunn, Jose Mourinho’s continual insistence upon picking his best players is ‘making mugs of Chelsea’s title rivals’.

Indeed, Thibaut Courtois, Branislav Ivanovic, Cesc Fabregas, Oscar, Eden Hazard, Nemanja Matic, John Terry and Gary Cahill have all made 16 starts or more across all competitions already this season, and all except the playmaking Brazilian have started every Premier League fixture.

There’s certainly nothing wrong with that and the Blues certainly boast impressive strength in depth. Courtois understudy Petr Cech is probably the greatest goalkeeper in Premier League history after Peter Schmeichel, Andre Schurrle just won the World Cup with Germany, netting three times at Brazil 2014 and summer signing Filipe Luis reached a Champions League final with Atletico Madrid last season.

Let’s not forget how well proven the likes of Ramires, John Obi Mikel, Loic Remy and Didier Drogba are too.

But the b-string have struggled when called upon this term. Kurt Zouma, Nathan Ake, Mohamed Salah, Luis, Drogba and Schurrle limped their way past League Two side Shrewsbury Town in October, with the 2-1 Capital One Cup result eventually decided by a Jermaine Grandison own goal.

Similarly rotated line-ups also struggled against Schalke and Maribor in the Champions League, drawing against the former at Stamford Bridge and the latter in Slovakia.

There’s no need to sound the alarm bells just yet and Mourinho claims no players will be going in or out of Stamford Bridge in January.

But a harrowing injury crisis could change the Chelsea landscape dramatically. Currently, there’s no real stand-in for Eden Hazard, who has featured in every Blues fixture this season, whilst the likes of Mikel and Ramires offer a completely different brand of midfield service to forward-thinking playmakers Cesc Fabregas and Oscar.

With Chelsea - and indeed, most heavyweight Premier League sides - undergoing a match every three-to-four days between now and the end of January, whether the Premier League leaders are anything more than simply an astonishing starting Xi will come to light over the next few weeks.

Battling on all four fronts

Chelsea’s dependence on key players would be no issue if they were solely competing for the Premier League title this season, but with one of the most impressive starting Xis, competent squads and coveted managers in world football, the west Londoners will be expected to compete for all four honours available to them this season.

Chelsea have made relatively light work of it so far as they remain undefeated, barring a few hairy showings in the Capital One Cup, but such enormous expectancy in all competitions will eventually catch up with them - especially whilst Jose Mourinho continually refuses to rotate certain sectors of his squad.

Indeed, fans may want the Premier League crown but the Champions League is Mourinho’s more favoured competition. After the New Year, when the knock-out rounds take place, the Portuguese will find himself forced to compromise certain fixtures and depend on players that have been ostracised in the first half of the campaign.

Likewise, Arsenal will likely drop out in the Champions League’s round of 16, Liverpool have too many domestic problems to progress significantly and anything past the Group Stages is now a bonus for Manchester City. But Chelsea possess the quality, experience and guile to go all the way and many, including Mourinho himself, will expect them to.

Then there’s the issue of the FA Cup - yet another competition well within Chelsea’s grasp that has a knack of complicating the fixture schedule.

Lack of killer instinct

It was evident enough against Sunderland on Saturday afternoon and had been pointed out by many pundits, including Gary Neville, previously - Chelsea lack the killer instinct of champions.

Despite possessing what is widely considered to be the most talented and organised defence in the league, the Blues have only kept four clean sheets out of a possible 13 this term, conceding on eleven occasions.

Most significantly, they threw away vital wins over Manchester City and Manchester United to draw both fixtures 1-1, with opposing goals coming in the 85th minute and 95th minute respectively. The Blues also surrendered a clean sheet in stoppage time against Crystal Palace, with Frazer Campbell finding a late strike.

Likewise, only against 19th-place Burnley and 15th-place West Brom have Chelsea wrapped up a result by half-time, in terms of boasting a two-goal margin at the interval. The vast majority of their goals have come in the second half after a rather tense first 45 minutes.

It’s not as if Premier League sides haven’t won the title with this kind of lapsed attitude before.

But Arsenal’s ‘invincibles’ often won the majority of their matches in the first half an hour, steamrolling opponents with three or four goals, and Sir Alex Ferguson’s Manchester United were famed for their ability to either hold onto a lead or push for a winning goal in the final minutes.

Chelsea on the other hand, thus far, have proved themselves proficient at neither.

The rivals are improving

A few weeks ago, it seemed as if Chelsea would stroll their way to the Premier League title uncontested. But let’s have a look at the league table now - and most importantly, recent form:

Although Chelsea are still enjoying a six point cushion at the league’s summit, the table is looking far more traditional than it did at this time last month, with Manchester City and Manchester United particularly coming back into form.

Challenging for the title may be a little beyond the Red Devils this year, but they’re finally beginning to play close to their best and that alone piles the pressure on the Blues.

Similarly, City have shaken off their cobwebs and put in a sensational ten-man performance against Southampton yesterday afternoon, recording a 3-0 win against the fourth-place side, suggesting they’re finally about to hit top gear.

Then there’s Arsenal, who will unquestionably improve when the likes of Mathieu Debuchy, Laurent Koscielny, Jack Wilshere and Mesut Ozil return to fitness after January.

Right now it might be plain sailing, but City, United and Arsenal’s revivals are either starting now or just around the corner. Suddenly, Chelsea’s six-point lead doesn’t look quite so seismic.

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