Lukaku is an experienced player. And he can play very fast. He has also played in the EPL before. He is aware of the EPL's style of play and strategy. Lukaku will not need much time to adapt to the team. Maybe he will be able to see his magic from the first match. If the understanding between Sadio Mane and Romelo Lukaku is good, Chelsea will be able to give their fans some great games next season.
It doesn't matter if you are a good player or not, it doesn't matter if you played here before or not, only thing that matters is here and now, nothing else matters. Obviously Lukaku is a great player and obviously he is going to help a ton, but if he is not comfortable this season, it is nothing like anything prior. If he has a fight with a friend, if he has a fight with the manager, if he has a problem on twitter while he was drunk and tweeted something, everything is a bit weird "could be ruined any moment" thing.
This is why I honestly think that we should not be focusing anything on this, we should be focusing only on the fact that Lukaku is a player who worths the risk, and if everything goes great than Chelsea will be very lucky, if not then they are going to be left over with a huge bill on their lap and nothing to show for it.