Chelsea have agreed a fee in the region of €13-15 million ($13.5m - $15.6m) for Strasbourg center-back Mamadou Sarr, sources told ESPN.
Enzo Maresca's side have found themselves in the market for for defenders, following injuries to Wesley Fofana and Benoît Badiashile. The club were considering a move for Crystal Palace captain Marc Guéhi before opting to recall his defensive partner Trevor Chalobah, who was on loan at Selhurst Park.
Sarr has been a mainstay in the Strasbourg defence, making 14 appearances for the club this season across all competitions.
Transfer dealings between the two clubs have come under scrutiny before owing to the fact that BlueCo, the consortium that owns Chelsea, are majority shareholders in the French side.
Last summer, Chelsea pulled the plug on a deal to send Deivid Washington to Strasbourg following concerns raised by FIFA and the Premier League. Diego Moreira's €9m move from the west London club to the Ligue 1 side was under review for similar reasons.
Meanwhile, Chelsea forward João Félix has emerged as a top target for AC Milan following their failed pursuit of Marcus Rashford. Felix joined Maresca's side for £46.3m in the summer but has struggled for gametime this season.