There was some confusion about what the club would do with the youngster, but he will return to France temporarily.
However, Chelsea have several defenders on their books, and Sarr is surplus to requirements, forcing him to return to Strasbourg.
He will continue to develop under manager Liam Rosenior, as they aim for a top-five finish.
Sarr made only one substitute appearance for Chelsea during the FIFA Club World Cup.
Levi Colwill, Benoit Badiashile, Tosin Adarabioyo, Wesley Fofana and Trevoh Chalobah are ahead of him in the pecking order.
Chelsea still have Axel Disasi, Renato Veiga, and Aaron Anselmino on their books. There is no way in for the young Frenchman.
He will be joined in Strasbourg by playmaker Kendry Paez and goalkeeper Mike Penders.
There is a theory that Chelsea wanted the trio to be around the squad before moving to France to avoid a situation where the players had no connection to the club before leaving.
The Blues signed Thibaut Courtois as a teenager, but he spent three seasons at Atletico Madrid on loan without getting familiar with Chelsea.
He was keen on staying in Spain, but the Blues refused and brought him to London in 2014, where he remained until he departed with a year left on his deal in 2018.
Chelsea are plotting a defensive shuffle with Disasi and Veiga on the chopping block. Both men are garnering transfer interest.
Anselmino will head out on loan. He was supposed to go to Strasbourg, but the club will have to find him another temporary stomping ground as they have filled their loan spots.
