Chelsea's New Year's day defeat to Tottenham was unprecedented. Many assumed Jose Mourinho's men would take the humbling in their stride and move past the humiliation at White Hart Lane. However, this has not been the case, with the Blues showing obvious signs of struggle and fatigue.
Notable absentees Diego Costa and Cesc Fabregas have been immense for Chelsea this season, directly contributing to 21 goals and 22 assists combined. Without the pair, Chelsea have look considerably weakened.
Costa has been a huge goalscoring threat for a Chelsea side that have not had a true goalscorer in past seasons. A significant upgrade on Fernando Torres, Costa constantly works to score goals for Chelsea, netting 17 goals this season and leading the race for the Premier League Golden Boot.
Even when not in goalscoring form, Costa makes his presence known and creates space for the likes of Willian, Eden Hazard, Fabregas, and Oscar. A thorn to opposition defences, Costa always makes a nuisance of himself.
Mourinho's backup strikers, Didier Drogba and Loïc Rémy, have been effective, but not as influential to Chelsea as the £32 million signing from Atletico Madrid.
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Fabregas works at the heart of Chelsea's midfield, keeping things ticking while constantly trying to push his team forward. His presence has been so important for Chelsea this season that without him, Chelsea are seen giving away possession or executing incomplete passes.
Indeed, without Fabregas, Chelsea look much sloppier in midfield. Without Fabregas, Chelsea's defensive-midfield looks considerably weakened, with John Obi Mikel, Ramires, and Ruben Loftus-Cheek the only other options left in that position.
Despite missing two of his best players, Mourinho's problems run much deeper. Ever since the turn of the year, and even before, Chelsea's once rock-solid defence has looked far too penetrable.
Problems in defence
Gary Cahill's form has dropped and, despite Kurt Zouma's solid performances, Chelsea's backline doesn't look as solid as it once was.
Branislav Ivanovic has been a key player for Chelsea, but his forays forward often leave Chelsea exposed, leaving many thinking that he could encounter more success in the centre-back position.
Cesar Azpilicueta does admirably on the left-flank, but would be much better on the right, as he is not naturally left-footed. Additionally, Filipe Luis has been a quality full-back, but his chances come at a premium.
Muddle in the middle...
In midfield, Nemanja Matic has been unable to completely dictate the tempo of the game, as he did at the beginning of the season. Fatigue largely contributes to this, but his backup options are not on the same level as him. Mikel and Ramires are good backup options, but, in terms of quality, Matic is on another level.
Chelsea's defensive problems need to be addressed by Mourinho as soon as possible if the Blues look to complete their dream of a historic treble.
The Premier League is Chelsea's to lose, while the Capital One Cup is almost in their grasp, and the Champions League dream would not be impossible to achieve if progression past Paris Saint-Germain is achieved.
With bigger matches on the horizon, Mourinho needs to revitalise his squad.