I don't completely agree with you because the performance of Lukaku is this season is not good enough to say he worth trying hard and paying a huge amount of money for him, Lukaku was injured in many games and he couldn't help his team for weeks, however after getting recovered Lukaku could show in the last week but all we saw from Lukaku was just in a few last weeks which is not enough to say he had a good performance.
It is true that Lukaku is a risky transfer and even though his statistics are alright, it is as you said. Injuries are a problem but also instability in his performances. But the transfer fees are at new heights anyway. When €40 million was record breaking a decade or 15 years ago, it is almost nothing these days. Usually I would agree and say that it is an expensive transfer given how much money Chelsea wants and how Lukaku did perform last season or was injured, but what if those sums are the new normal? Is €40 million still too much?