When Chelsea face Everton on Saturday evening, the Blues will be reacquainted with one they let slip. A player whose goal scoring record will forever be quoted at them, as will the fee they received.

Romelu Lukaku arrived at Chelsea as the next great hope. Warranting a hefty transfer fee after impressing for Anderlecht, the Belgian was meant to be the first in line to King Didier Drogba’s throne. Spending one season watching and learning from the great Ivorian, Lukaku was expected to be set for a first team spell. Instead, he was loaned out soon after Chelsea’s against-all-odds Champions League triumph and never made a mark at Stamford Bridge.

The eventual, permanent move to Everton in summer 2014 shocked many. Consistent goal scoring exploits whilst on loan, paired with Chelsea’s appalling striking form, made it a no-brainer for Lukaku to return and lead the line. Rather, the club opted to cash in on one of football’s most valuable assets. Since then, Lukaku has continued along a similar line of progression – netting seven league goals already this season.

Chelsea's saving grace, however, has been Diego Costa. Costa’s arrival lessened the blow of the Lukaku sale and made it sink into irrelevance when the former Atletico Madrid man led Chelsea to the 2014/15 Premier League title.

This is where the greatest uncertainty lies though – was it possible for Chelsea to retain Lukaku and sign Costa? If so, why did they not do exactly that?

Lukaku may have not wanted to play second fiddle to the Spanish international, and it's fairly natural that he may not have seen a future for himself at Chelsea in that situation. It's understandable, but Chelsea left themselves drastically short as a result. When Costa was short of form and regularly injured last season, Lukaku could have helped shift the Blues out of their rut. Everton, relatively speaking, picked up a bargain and - amongst all the strange goings on at Stamford Bridge - the sale of the Belgian striker remains one of the hardest to fully grasp.

Whatever the root cause of Lukaku’s transfer to Everton, Chelsea are worse off for it. They may not have won the 2014/15 title with Lukaku rather than Costa, they may not even have been in contention, but they would be set up in the long term. Patience is a virtue severely lacking when it comes to Jose Mourinho and Chelsea as a club, but it is the current Manchester United manager who is responsible for the fact that Chelsea sold a player with the potential to become a club legend.

There is still an immaturity present in Lukaku’s game, however, and certainly compared to the canny experience of someone like Costa, the Belgian can come across as crude. Devastating on a fast break, Lukaku’s ability as an all-round striker is somewhat undermined by his ability to link up with his teammates. Whether his improvement as a more rounded footballer would have been quicker at Chelsea is an unknowable hypothetical, unfortunately. At the time, introducing Costa for Lukaku was an obvious short-term decision. Even now, Costa - the Premier League's top scorer - has the upper hand. It is, painfully for Chelsea, after a few years that they will be struck by the error of their ways.

For all his limitations, Lukaku continues to stagger. His age, physical prowess and calmness in front of goal make him destined for greatness and Chelsea will never quite move on from what could have been had he remained at the club. A goal at Stamford Bridge is almost inevitable on Saturday and a negative result for Chelsea will only add gallons more fuel to the Lukaku fire.

Roman Abramovich and his decision-makers at Chelsea now have their eggs in the Batshuayi and Bertrand Traore baskets, and will hope they hatch before Lukaku becomes one of the world’s best.