The Ghanaian is expected to respond better to rehabilitation with the west London club who have dealt with his complex injury problems in the past.
Real's assistant manager, Aitor Karanka, has hinted that Essien's failure to respond to treatment in Spain is the main reason for his return to Stamford Bridge.
"Essien is not injured but is suffering from an allergic reaction to a drug," stated Karanka at a press conference.
The 29-year-old has struggled for sustainable fitness in recent years and only managed 14 appearances for Chelsea last season. However, the Liga side are optimistic that his latest problem can be quickly resolved and he can return to Madrid in the New Year.