When Kevin De Bruyne departed Chelsea last summer for Wolfsburg in a deal worth £18 million, it was very much clear that, for Jose Mourinho, the Belgian was surplus to requirements at Stamford Bridge.
The talented midfielder featured very little in west London, such was the competition for places and Mourinho's inclining to favour defensive-minded players.
Consequently, to the detriment of Mourinho, there was no space for De Bruyne in his philosophy.
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However, with Eden Hazard and Cesc Fabregas going largely unnoticed this season, the Portuguese must rue the sale of De Bruyne, especially for what is now such an inadequate fee.
After leaving Chelsea for Wolfsburg, the newly invigorated De Bruyne won the illustrious Player of the Year award in the Bundesliga. Thereafter, Manchester City recognised his immense talent and bought his services for a club record fee of £55 million.
The 24-year-old has enjoyed a bright start to life in Manchester, each game conveying his vast quality through the five goals he's scored in nine games for the Citizens.
Back in west London, the dynamic duo of Diego Costa and Fabregas have both been absent in form and quality. Costa has just three goals to his name in all competitions this season - a far cry from the clinical striker that tore apart defences last season.
It's very difficult to determine why exactly Chelsea are struggling. Key players are a shadow of their former selves and failing to inspire upon entering the field of play - all except for the Brazilian Willian, that is.
His direct style of play, crossing ability and free-kick prowess have all been on show recently, but unfortunately for him, his team-mates aren't reciprocating such efforts.
One can't help but feel that, if Chelsea had nurtured and developed De Bruyne amongst their ranks, they could have had a Frank Lampard-esque player on their hands - creative, clinical and full of energy.
De Bruyne, simply, is everything Mourinho is missing this season. His attacking exploits have received such praise that he warranted a place in this year's Ballon d'Or 23-man shortlist - a remarkable achievement for someone who was largely an outcast prior to last season.
His first touch, pace, dribbling and range of passing are quintessential attributes that a world-class midfielder must ascertain to become a key component in a title and Champions League winning team - if only Mourinho had kept faith with him to reap such benefits amidst his struggle get his team firing once again.