I've mentioned times without number that the problem with Chelsea lies on their coach, I feel like he lacked the ability to effectively manage the players at his disposal. I don't think it is true to say that Chelsea lacks the squad depth they require to challenge the status quo but lacks the ability for effective management of those players. We have the likes of Romelu Lukaku who haven't been used effectively yet a lot of money have been spent on his transfer and so on.
Chelsea were able to finish in 6th position thanks to the help of Pochettino's contribution after previously ending in 12th position which looked worse but the coach has the enthusiasm to give the best for Chelsea but this club owner is the one who is a problem for his own club after succeeding in reaching the best point but instead fired Pochettino.
Before
After 
Source :Premier league points table
When you say about Romelu Lukaku as a player who can be relied on at Chelsea but the club does not choose to use him as a main player, the reason is simply that Chelsea knows that Lukaku actually does not have much interest in appearing with Chelsea and prefers to be with a club from Serie A.