Umtiti had been having treatment with specialists in Qatar ever since the end of November.
He has played just seven times in La Liga this term, having also missed two months when the knee injury first flared up in September.
The club are yet to give a return date for Umtiti, though they will be helped by the loan signing of Jeison Murillo in January.
A club statement on Umtiti's return to Spain read: "Samuel Umtiti was evaluated by the club's medical services staff at the team's training centre this morning due to the discomfort in his left knee.
"Samuel Umtiti will return on December 30 to continue undergoing conservative treatment on his knee.
"This conservative treatment plan was announced on December 2, with the goal of allowing him to heal his left knee discomfort.
"The player was in Doha, Qatar in recent weeks for physical therapy treatment under the supervision of the club's medical services staff."