Given that everyone has heard Lukaku's many comments bout wanting to play in the champions league and win trophies, its pretty certain that the new contract that he signs will have a release clause.
It guarantees Everton a minimum transfer fee, and avoids a long drawn out saga. If Chelsea want him, Everton will say, meet the buy out clause - end of discussion. Hopefully that allows any deal to happen quickly, allowing us time to find replacements.
Not sure how it works if someone meets the buy-out clause a day before the window closes and Lukaku wants to go. There must be a provision in the contract to avoid that happening.