That depends on how progress between lukaku and chelsea. I just read from gazetta if wants to sell him as soon as possible. Chelsea has been even decreasing lukaku's price so much. He is not priced almost a half from previous price.
Chelsea are a rich club but they must obey financial rules from UEFA. They are already in a watch list of UEFA and they will have to try to balance between transfer values from bought and sold players. With players who are receiving high salary and Chelsea don't have plan to use, they will try their best to sell those players and get some sort of money back. Lukaku is in their sale list, no doubt about it.
Chelsea are undoubtedly a rich team, but giving away talent is never a good thing. Of course if he has to raise cash it's another matter, if it were me I wouldn't easily give up a player like Lukaku who can make the difference in the opponents!
Chelsea's performance last season was very poor. And so they want to rebuild the team. One of the reasons for Chelsea's erratic performance was the poor bonding between the players. So Chelsea's new coach is keen to sell old players to form the team he wants. I expect Chelsea to perform better next season than last season even after selling Lukaku.