The Blues will finance the deal with funds raised from the sale of Kevin De Bruyne to Wolfsburg with Jose Mourinho keen to bolster an under-manned area of the squadBy Wayne Veysey
Chelsea are on the brink of re-signing Nemanja Matic from Benfica in a deal worth €25 million.
Talks will be held in Lisbon on Monday in an attempt to finalise the Serbia international's transfer back to London.
Chelsea are believed to have made a bid of €23m rising to €25m in add-ons for Matic, 25.
The midfielder first moved to Stamford Bridge in 2009 for less than €2m and was then sold to Benfica two years later as a makeweight in the deal that brought David Luiz to the club.
It represents a huge mark-up for the west Londoners but manager Jose Mourinho regards Matic as the type of midfield anchorman the team has been lacking for several years.
Tall, athletic and comfortable with the ball at his feet, the Serbian will be expected to slot straight into Mourinho's side if the deal can be completed in the coming days.
Chelsea are said to be confident of agreeing a deal with Benfica and imminently announcing Matic as their marquee signing of the January window.
The Europa League champions expect to use the funds from Kevin de Bruyne's sale to Wolfsburg, who on the verge of a €18m move to the Bundesliga.
Mourinho has been keen to buy a proven defensive midfielder since returning to Stamford Bridge last summer and finding a squad overloaded with playmakers and light in the central areas.
With defender Luiz regarded as a last resort in midfield, Frank Lampard out of contract in the summer and Marco van Ginkel sidelined for the season with a cruciate ligament injury, strengthening in that area has been regarded by the Blues as a priority.