Chelsea have been engaging in talks with Sporting CP for defender Ousmane Diomande.
The young center half, who is only 20, is a target for several top European clubs.
However, Chelsea are hoping to steal a march on their rivals to secure Diomande's signature.
Trevoh Chalobah could leave Stamford Bridge this month, freeing up a squad spot in defense.
Whether Diomande is available this winter is another matter, as Sporting are in no mood to weaken their squad mid-season.
The Blues may have to wait until the summer window to secure their top defensive target.