Chelsea boss Frank Lampard has reportedly been told that he must sell one of his central defenders before being able to pursue a deal for a replacement.
While the Blues have impressed going forward in recent weeks, they have failed to improve in defence, leading to 54 goals being conceded in the Premier League.
According to The Telegraph, that has placed club officials under pressure to find a buyer for one of Lampard's current centre-backs.
With Antonio Rudiger likely to be retained due to his role in helping Timo Werner settle into life in England, it effectively leaves Andreas Christensen, Kurt Zouma and Fikayo Tomori available for transfer.
Signing a player who can play in the middle and at left-back remains an option, providing that Emerson Palmieri is moved on during the summer window.