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Chelsea has prematurely let go of some brilliant players over the years – Mohamed Salah and Kevin De Bruyne, to name but two.
Letting go of Romelu Lukaku, however, might be the club's biggest ever transfer mishap.
Since leaving Stamford Bridge permanently in 2014, Lukaku has scored 175 goals in just 317 games for Everton, Manchester United, and Inter Milan.
27 of those have come for Inter this season, putting the club to the brink of its first Serie A title since 2009/10.
The often-lamented striker has also improved vastly in terms of his technical ability at Inter under Antonio Conte, who, ironically, tried to re-sign the Belgian during his stint as Chelsea's manager.