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Chelsea have signed Nemanja Matic from Benfica on a five-and-a-half-year contract according to the club's website.
The 25-year-old, who arrived in the capital last night in order to finalise his Benfica exit, is thought to have joined for around £21m.
The move comes only three years after Matic left Chelsea, when he was used as a makeweight in the deal that brought David Luiz to Stamford Bridge.
However Jose Mourinho reportedly marked the midfielder out as a key transfer target when he also rejoined the club and the Chelsea hierarchy clearly had no problem with signing a player they let go so recently.
On signing his five-and-a-half-year contract, the Serbian said: "I am very happy for this opportunity to be back at this club. I feel very good because of that and just want to say I will give my best to help the team and make the Chelsea fans happy."
Jose Mourinho added: 'I am very pleased that Nemanja is returning to Chelsea. He has grown as a player in Portugal and has become a fantastic all-round midfielder. I am sure he will become a very important member of the squad and help us to achieve our ambitions.'
It is thought that Chelsea will offset most of the cost of the transfer with the imminent sale of Kevin De Bruyne to Wolfsburg for around £16m.
Show all 18 1 /18Chelsea - January transfer targets Chelsea - January transfer targets Luke Shaw - Another potential long term successor to Ashley Cole, the Southampton youngster has impressed everyone and would surely command a fairly hefty fee given his side's recent success.
Chelsea - January transfer targets Sami Khedira - Currently out with a long term injury so a January signing looks unlikely but Jose Mourinho is said to be keen to link up with his former charge and the German is just the sort of player Chelsea need in midfield.
Chelsea - January transfer targets Fredy Guarin - At one point very close to signing with the club, the £15m Colombian, cooled for a while. However, with the move to Juventus, stalling - Chelsea may with the use the funds from Mata's transfer to Man Utd and make another bid.
Chelsea - January transfer targets Andrea Ranocchia - Chelsea are one of a few Premier League clubs chasing the Inter defender and you can see why, the tall centre-back is a handy player.
Chelsea - January transfer targets Claudio Marchisio - Juventus' early Champions League exit may leave them a little strapped for cash with Marchisio being mooted as a player to be sold in order to cover the shortfall. Chelsea would no doubt be interested in his midfield class should he become available.
Chelsea - January transfer targets Eliaquim Mangala - Another defender Mourinho is reportedly very keen on, the Porto centre-back would not come cheap although his club have admitted they would be willing to sell him for the right price. Manchester City have since shown an interest however.
Chelsea - January transfer targets Luis Suarez - Chelsea need a goal-scoring striker, so who better than the league's top scorer? A very unlikely move for a lot of obvious reasons.
Chelsea - January transfer targets Diego Costa - The Atletico Madrid man is another striker in red hot form and consequently he has attracted the attention of several Premier League clubs. Mourinho would no doubt enjoy swiping him from under Arsene Wenger's nose.
Chelsea - January transfer targets Xabi Alonso - Another of Mourinho's former players who would add experience and quality to Chelsea's midfield. Given his age shouldn't prove too expensive.
Chelsea - January transfer targets Radamel Falcao - Linked with Chelsea for some time, the free-scoring Colombian would cost a fortune to prize away from mega-wealthy Monaco, but the promise of Champions League football would surely be enticing.
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Chelsea - January transfer targets Edinson Cavani - Another big name striker who would cost a lot to sign, this time from PSG, Cavani has again consistently been linked with a move to Stamford Bridge.
Chelsea - January transfer targets Hulk - Apparently fairly disenchanted at Zenit and another long time Chelsea target, nevertheless would seem an odd purchase given his usual berth is out wide - a position Chelsea have a ludicrous amount of depth in.
Chelsea - January transfer targets Jackson Martinez - Another Colombian reportedly on Chelsea's radar, the Porto forward has an excellent recent goal record but might represent more of a risk given he would still command a large transfer fee. With Juan Mata now in Chelsea's past, a new midfielder is needed.
Chelsea - January transfer targets Gonzalo Higuain - Seems unlikely given he only moved to Napoli in the summer, but Italian media are reporting that Chelsea are interested in the Argentine forward. Another of Mourinho's former men.
Chelsea - January transfer targets Wayne Rooney - Chelsea's main target over the summer and despite apparently being happier at Old Trafford has yet to sign a new contract. Failure to qualify for the Champions League would surely see Rooney depart. Will selling Mata to the champions make them loosen their tough stance on the England man?
Chelsea - January transfer targets Fabio Coentrao - With questions over Ashley Cole's place in the side, Chelsea are reportedly having a look at another Mourinho old boy, Real Madrid's Coentrao.
Chelsea - January transfer targets Mario Balotelli - Is a Premier League reunion with Super Mario back on the cards? That's what sections of the Italian press think. It seems unlikely given his previously very poor working relationship with Mourinho but you just never know.
Chelsea - January transfer targets Alberto Moreno - The young left back is another being touted as Ashley Cole's successor - with a buy out clause of £25m he wouldn't be cheap but then aged 21 he has a long future ahead of him.
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