Chelsea did not include a buy-back option for Fiyako Tomori.
The Blues academy graduate joined AC Milan this summer after a successful loan at San Siro last season.
Milan paid €29m for Tomori, who they consider "untouchable", according to Fabrizio Romano.
Tomori started for the Rossoneri in Wednesday's loss to Liverpool in the Champions League.
The 23-year-old is considered an important player in Stefano Piolo's side.
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