A big problem position for the Blues -- Rafael Benitez continually opted for a double-pivot last year in a 4-2-3-1 in order to get the most out of Oscar, Eden Hazard and Juan Mata. The problem is that only John Obi Mikel is really suited to playing in such a role -- Chelsea need to either branch out with their playing style, or find someone suitable to play in that position. Either way, central midfield is top priority.
Has excelled for both Real Madrid and the German national team in a double pivot, and would be a good partner for Mikel. There is a suggestion that Real Madrid will let him leave, although there has been little concrete to give reason to believe that a move is on the cards right now. Nonetheless, a player of his ilk would be ideal.
A more cerebral option who would bring something different to the role, Modric proved at Tottenham that he can play in the middle, but much like Khedira, it's doubtful whether Madrid will let him go, despite the perception in Spain that he's been a flop.
Daniele De Rossi
A sort of halfway house between the two above, De Rossi would add some of the steel and drive of Khedira combined with some of the playmaking of Modric. Chelsea have reportedly contacted Roma over a deal, but it's very unlikely that the club would be willing to sell. If they were serious, the issue could be forced, but with other positions in need of strengthening, it'd be a difficult fight for Chelsea to force a sale.
With Fernando Torres being Fernando Torres, and Demba Ba having been rather disappointing, a good forward presence is vital for Chelsea both to add a real goal threat and aid the three midfielders behind him. The bad news is that there are rather a lot of clubs looking for such a player right now.
A complete center-forward, well-versed in operating as a lone striker, and a prolific scorer of goals, Cavani would be a no-brainer for any side, let alone one he would fit into as effortlessly as Chelsea. Manchester City will put up some fierce competition, and may have more clout in getting a deal done, particularly with Napoli's rumoured interest in Edin Dzeko, but the Blues are very much in the race here.
Less of a fit than Cavani, but comes with the added bonus of being able to feature in place of one of Chelsea's attacking midfield three, where he could offer as much as anyone. This deal is also looking very likely, with Bayer Leverkusen's chairman confirming that it would go ahead if, as is very possible, the Blues permit Kevin de Bruyne to move on loan in the other direction.
Like Cavani, Rooney provides a well-rounded presence up front, and like Schurrle, could also play deeper if required, but there are caveats. Firstly, his vast wages; secondly, persuading United to sell to a Premier League rival (likely paying a premium in the process); and thirdly, signing a player who is giving every impression of being on the wane and with several fitness problems. Chelsea have signed one striker in that mould before, and doing so again would be a gamble of extraordinary proportions. Of course, whenever he's played as a straightfoward striker, Rooney has bagged plenty of goals, although this one seems to have gone cold recently. Still an option nonetheless.
The third big need for Chelsea, with Gary Cahill showing little to suggest he can be a part of a title-winning defense, John Terry finding himself out of the team increasingly often, and Branislav Ivanovic proving that despite his physicality, he is very much not a central defender.
A hugely talented defender, but Mangala would represent a significant outlay for Chelsea, which they may struggle with if, as is likely, they spend big on the above two positions. Porto's strategy has generally been to sell one or two players each year, and with James Rodriguez and Joao Moutinho already having departed to Monaco, they will be desperate not to avoid a further exodus from the club. Next year may be a better time to make this particular move.
Lovren has been linked to a number of top clubs for some time, deservedly so, and this summer may be the time that Lyon finally lose their grip over their key center-back. A very well-balanced and all-round defender, he should be able to slot into the Chelsea side quite quickly, and rumoured Atletico Madrid interest aside, it should be a pretty straightforward deal if the Blues decide the Croatian is their man.
Marquinhos has one season in a dodgy defence in Serie A to his name in terms of really judging his performance, but it was an utterly phenomenal season by anyone's standards, let alone someone so young. He could easily be one of the top handful of defenders in the world very quickly given his tender age, although may need time to adjust to the Premier League. He would be a gamble in the short-term, and Roma will be reluctant to sell, but it is certainly feasible.
Marco Van Ginkel
A fine Dutch prospect and a versatile attacker who can play across the midfield, Van Ginkel is nonetheless not quite ready to be thrust into a top Premier League side right now. There could be competition from Manchester United, but if the midfielder is bound for Stamford Bridge, it likely won't be for very long. This one has 'Bundesliga loan' written all over it.
Southampton claim to be confident about keeping their precocious left-back, but when he's been referred to as Ashley Cole's future replacement, it may not be just for the national team. Nonetheless, with Cole still playing well and Ryan Bertrand around, it may make sense for Chelsea to bide their time before making a move, unless interest from other clubs forces their hand -- many have also been linked, among them Manchester United.
With Thibault Courtois set for another loan, Petr Cech not going anywhere, and Ross Turnbull and Hilario being released on frees, Chelsea need a reserve goalkeeper. Few names have been mentioned so far, with Diego Lopez probably only mentioned due to the Jose Mourinho link. It's unlikely that big money will be spent here with Courtois available in future.
Kevin De Bruyne
There's a suggestion that Mourinho has plans in the first-team for the Belgian, but he could be shifted to Bayer Leverkusen on loan as part of a deal for Andre Schurrle. Either way, the notion that Borussia Dortmund would buy the player outright now seems to be dead.
After a very mixed season at Real Madrid, Chelsea will have to find something to do with Essien, whether it's loaning him out again or using him in their squad. They could also find an outright buyer -- the likes of Monaco could be tempted. He was an effective player under Mourinho at Real Madrid, so he might not be off after all.
Rafael Benitez may well have been interested in bringing the Brazilian to Napoli, but with Chelsea's defensive struggles, it's hard to see they'd entertain the idea of letting their best centre-back leave the club. A non-starter.
Another player linked to Napoli as part of a deal for Cavani, but the Partenopei almost certainly couldn't afford his wages, and those that could could probably do a lot better for themselves. Will probably still be at the club next season.