Chelsea are already the strongest and most complete side in the Premier League in terms of their 1st 11 and their depth, therefore it is scary to believe they could actually get better. But believe you should; the Blues have a world class player in each position except two: attacking midfield and the right wing. Secondly, Chelsea have an able deputy in every position except defensive midfield.
In regards to their attacking midfielder, that position is regularly filled by Oscar. The 23 year old is steadily improving year upon year and it is only a matter of time before he has his breakthrough season as a consistent world class performer.
As for the right wing, Willian is normally deployed there. Like Oscar, Willian is a very good player who is among the league's best. He does not, however, have the potential to progress much further than what he is right now.
Finally, in defensive midfield, Matic's deputy is John Obi Mikel. Mikel is a solid defensive midfielder in his own right but simply lacks the intensity and creativity Matic gives them. Opposition clubs spot him as a weak link the side and too often, instead of being Chelsea's protector, he is an instigator for trouble. It comes as no surprise the only game Chelsea have lost occurred when Matic was out with suspension and Mikel deputised.
Without reinforcing these positions, Chelsea will still probably win the Premier League, such is their advantage over everyone else; the Champions League, however, is a different kettle of fish and Chelsea will need to exercise every opportunity needed to win it.
Below are the players Chelsea should sign in order of importance:
1) Defensive Midfielder
As long as Matic is available to play, defensive midfield is not a major problem. The issue comes in his absence. Chelsea have simply outgrown Mikel's services and it is time they got an upgrade. However, there is the additional worry of Fabregas' suitability in a deeper role. Fabregas has been great from an attacking sense playing in a deeper role this season. His 13 assists so far this season represents the highest amount of assists to be registered at this stage of a season and he looks set to break Thierry Henry's all-time Premier League record of assists (23). Nonetheless, defensively he is a liability. Against Liverpool, Fabregas was given the run around by Coutinho and had Liverpool had more of an edge to them that day, the scoreline could have been different. This is supported by the fact Fabregas has been dribbled the most out of any Premier League player this season. This has yet to prove costly in the Premier League but will result in a knockout in the Champions League.
This is where Oscar comes in; Fabregas is the better player and should therefore be played as an attacking midfielder in the bigger clashes, whilst Oscar drops to the bench. Assuming Fabregas' old role should be a player full of energy, a player with technical ability and a player that is responsible in defence; that player is Sami Khedira.
Khedira has hardly featured for Madrid at all this season due to contract negotiations breakdown. Khedira is in his last 6 months of contract and has a favourable relationship with Jose Mourinho to boot. If Chelsea wish to be truly impenetrable, having a monopoly on two of the best defensive midfielders in the game would be a great start. To sweeten things, the transfer would rob Madrid of their only true defensive midfielder.
If Chelsea were able to sign Khedira, it would spell the end of Mikel's career at Chelsea. However, having been a respected member of the Chelsea squad for so long, Chelsea could break even on this deal.
As stated earlier, Willian is a very good winger, arguably the third best in the Premier League behind Angel Di Maria and Eden Hazard. However, the gap between Willian and the very best becomes more pronounced when you compare him to the very best in world football. Similarly, Chelsea's back up options, Andre Schurrle and Mohamed Salah have shown a lot of promise but have failed to perform consistently.
Clearly Chelsea can do better and luckily for them Marco Reus is available. Reus has resisted Dortmund's many advances to sign a new contract. As a result, Reus' contract with Dortmund will expire in June 2016 - in a year and a half. In the past Dortmund have fought the urge to sell a player on cheap - losing Robert Lewandowski on a bosman transfer after Bayern offered £30 million the previous year is the perfect example; however, the Reus situation is slightly different.
Reus has a £21 million release clause in his contract which will activate in June 2015. Whether Dortmund like it or not, they will lose their player for that figure should he wish to leave. Chelsea can take advantage of this a pay a bigger sum to Dortmund who will be armed with a load of cash and can try to rectify their ailing season.
In return, Chelsea gain one of the best wingers in world football and will be one step further to becoming a complete side full of world class stars. To sweeten the deal, Andre Schurrle could be sold in an exchange or sold to another club afterwards.
Aside from the aforementioned, Chelsea do not have to sell anyone else. Although, the Petr Cech situation is something that needs to be rectified sooner rather than later. Cech, a world class goalkeeper in his own right, is too good to be rotting on the bench. His agent has already been sounding noises about several clubs wanting his services. If Chelsea see fit, they could sell him now as their main Premier League rivals (Manchester City and United) do not need him and as he cup-tied for the Champions League too. As Cech is likely to play the 3rd round of the FA Cup, he will be cup-tied for that competition too.