I don't think Lukaku is irreplaceable. We might not score the same type of goals but we will get them a different way.
The problem we face is getting a replacement in straight away who can replace his goals and assists from day one. Look across the park, how many players did it take to replace Torres? And then again with Suarez, or even close to him?
I have no doubt there are players that can come in and do as well as or better than Lukaku but how many seasons will go until it happens?