OK here's the second of the positional break downs, I haven't altered the content very much either on this one, but I might look to do so more in the future, perhaps integrating some of the club based stuff at the bottom of ' Chelsea: The Transfer Market Story 2013 and beyond' .
Concerns around the future of John Terry aside, our central defence looks not too bad (rather like Right Back which has been considered here); David Luiz, Gary Cahill, Branislav Ivanovic, Jeffrey Bruma, Emir Dautović, Nathaniel Chalobah, Tomas Kalas, Andreas Christensen, Jordan Houghton, Nathan Aké, Fankarty Dabo, George Brady and Alex Davey. But there is always room for improvement, and central defence could see some additional strengthening to avoid Man Utd style injury crises. Ryan Shawcross has been mentioned recently, he's dependable, young and English and a leader on the field, but he's also not necessarily the best option.
Fairly immediate consideration should be given to the four (fairly) young German (and Germany based) stars; Neven Subotic, Mats Hummels, Koray Gunter all at Borussia Dortmund and Schalke's Benedikt Höwedes. Anderlecht and Senegalese star Cheikhou Kouyaté would be a fine buy, but there are also other impressive options out there, Basque trio Iñigo Martínez, Fernando Amorebieta and Jon Aurtenetxe (who can also play at left back) and in the youth ranks, long term Chilean-Polish target Igor Lichnovsky, Andrea Ranocchia, attacking Japanese centre back and Man Utd target Hideto Takahashi and Mexico youth star Antonio Briseno.
Personally I'd eventually like to add Takahashi for Asian merchandising purposes, Lichinovsky as a rising star and Kouyaté as a mentor for Chalobah. If we need to replace Terry we could go for Ranocchia or one of the German's (in this case that includes American-Serb Neven Subotic), or maybe Amorebieta.