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.
Soccer transfer history is full of faults like these so right now no one is thinking about past what happen and how they have done this mistake because it's all gone with mostly looking for the future and right now Lukaku is not having any desire for the playing under Pochettino and Chelsea is also looking for potential buyer with right now they have three bids from different clubs like Juventus, Roma and Inter most chances with the entry of Juventus things solved quickly and easily because right now their bid is best, but Chelsea wants £45 million and no one near this but as things are going most chances this will also be completed, and we will be having Lukaku in Serie A for the next season and his performance could be also improved because he needs to be back his best for having good future at the top flight of UEFA clubs in next few years.
If considering their transfer that cost a lot of money of course it will be a regret for Chelsea, but there is no point in regretting it anymore and they better focus on the future. Now will they want to let Lukaku go for a price that is far below when they brought him from Inter Milan, or will they insist on keeping Lukaku on loan.
For some reason when watching him play for Chelsea it seemed something was missing, and when he returned to Inter Milan he seemed more comfortable. I'm not going to speculate too much about this, because until now I haven't heard any bad news, or maybe it's because the Premier League's style of play doesn't fit or there are other things, like for example, it takes longer in the adaptation process.