Marseille president Vincent Labrune has told L'Equipe his side have received a "very serious" offer for the Spaniard.
He told ESPN: "I have spoken to Chelsea's chief executive and they are easy to deal with. We have told them the price and they will come back and agree it or not.
"We would much prefer the player to sign a new contract and stay but he wants to go to Chelsea and it's a good club so we cannot stand in his way."
Chelsea are also being linked with a renewed bid for Bayer Leverkusen's Andre Schuerrle having already spent almost £70m in recent months.