One of the biggest shocks of the summer transfer window was when Nemanja Matic decided to move from Chelsea to Manchester United.
Although he was not one of Chelsea's top performers, he had still been a reliable component in a title winning side.
However, Antonio Conte felt as if he was expendable, and decided to sell him to their rivals in order to fund the signing of Tiemoue Bakayoko.
Big mistake. Chelsea have been underwhelming this season, and have failed to cope with the absence of Matic in midfield.
To make matters worse, Man United have started this season very well, with their defence a key part of their early success. A key reason for this could be down to the arrival of Matic.
MATIC FEELS HE HAS NOTHING TO PROVE TO CHELSEA
Given his departure from the Blues, you would forgive Matic for possessing some sort of bitterness towards his former club.
However, he recently revealed that he has nothing to prove to his former club.
Speaking ahead of Man United's game against Chelsea on the weekend, Matic said: "I feel when I played there, I showed them my quality.
"We won the title twice and also two cups, so I don’t have to prove to them anything. I just have to prove something to my new club.
"When I played there I always gave 100%. I’m sure they respect that but if not I don’t really care.
"I am happy to be part of this club. I’m happy to be working with Mourinho.
"When I played at Chelsea I give always my best to help the team but now Manchester United is my club and I will try to win there."
THIBAUT COURTOIS BELIEVES CHELSEA WERE RIGHT TO LET MATIC GO
Although Matic was a big part of Chelsea's success, Thibaut Courtois has revealed his belief that the club had to let the Serbian leave.
The Belgian goalkeeper explained his opinion, stating: "If a player nowadays wants to go to a team, then he has – I don’t want to say the power – but if he has a strong will to leave and join another team then it is hard for the club.
"You cannot just ignore him and say ‘you cannot leave’ because then you have a disappointed player who won’t play at his best."
Courtois raises a great point, but given his success and their own woes, it is almost certain that Chelsea regret the decision to let the midfielder leave.