Shakhtar Donetsk coach Mircea Lucescu says potential suitors for winger Willian must match the player's release clause should they wish to sign the Brazilian.
Willian has been subject to interest from both Tottenham Hotspur and Chelsea in recent months, scoring twice against The Blues in the Champions League fixture last week.
And Lucescu says that while clubs must match Willian's €35m release clause, Chelsea must be looking to pay more than they splashed out for midfielder Oscar.
"If [Inter president Massimo] Moratti asks me about Willian, I will tell him that Shakhtar want to achieve things in Europe and will not sell their players. There's the buy-out clause, though, which is €35m for Willian," Lucescu told Tuttosport.
"Chelsea paid €31m [£25m] for Oscar this summer. The difference between Willian and Oscar is that Willian is much more mature, and has already plenty of Champions League experience and has played in a strong European league for years.
"The Blues wanted to sign Willian this summer for €20m [£16m], but we turned them down. They went for Hazard for over €30m [£24m] instead. Fact remains that we do not want to sell him."
With Fernando Torres and Daniel Sturridge the only recognised strikers in the Chelsea squad, Roberto Di Matteo may be tempted to add to his goal threat in the January transfer window, however the addition of Willian is likely to further swell his selection headache.
While the Italian has Eden Hazard, Oscar and Juan Mata as his three attacking midfielders, Di Matteo also has to consider with recent impressive form of Victor Moses and German Marko Marin, who is still to make an impact since his move from Werder Bremen.
In addition, after the club spent big in the summer to bring in Hazard and Oscar among others, and having spent £50m on Torres in the January of 2011, it is yet unknown whether owner Roman Abramovich will sanction another spending spree as Chelsea seek to get closer to the Manchester pair in the title race and become the first club in the Champions League era to retain the European Cup.