Maybe it was Chelsea's fault but the last chance was to sell Lukaku to Inter milan and be signed permanently because Inter milan really needed Lukaku to strengthen his club like the previous season.
I don't fully accuse Chelsea of doing anything wrong but there is no other choice but to sell Lukaku to fix the finances for $40 million it doesn't matter the most important thing is that Inter milan can get Lukaku back for next season.
We don't know what steps Chelsea will take next to earn $40 million now other than having to sell players like Lukaku. Because we also don't have to consider that as a mistake for Chelsea because Chelsea itself is also in need of the best players like Lukaku next season. Moreover, Chelsea is also trying to improve itself which has slumped last season by trying to get some great players to move better next season. So even if Chelsea don't sell Lukaku, it shouldn't be considered a mistake.