Blues boss Jose Mourinho is a fan of the France international having worked with him for two years during his time in the Spanish capital.
Mourinho had previously said that the club’s transfer business was concluded, but could make a bid amid growing speculation that the 21 year old could be sold by former Chelsea boss Carlo Ancelotti.
The former Lens man made just twenty-three appearances in all competitions last season, the last of which coming in the Champions League final victory over rivals Atletico Madrid.
He also appeared in all five of Les Blues’ matches at this summer’s World Cup finals in Brazil.
Chelsea have already brought in three players from the Spanish top flight this summer in the shape of former Atletico duo Felipe Luis and Diego Costa and ex-Barcelona man Cesc Fabregas.