The Frenchman moved to Stamford Bridge from Monaco last summer for a reported £40million, but struggled to make the desired impact in the Premier League.
Chelsea said in a statement: “The 23-year-old now makes the temporary switch to the Serie A side who are managed by Gennaro Gattuso and are competing in the Europa League this season.”
Milan said the deal includes an option to make the move permanent.
He will be presented to the media on Friday afternoon.
