You've got to wonder if Mourinho is sat at home pushing 14-year-old kids over in anger when he watches De Bruyne play.
And what a mistake that was! Not only did the Belgian tear the Bundesliga apart last season, but he managed to equal the league's all time season assist record.
Then he's moved to Manchester City for £50 million and now he's just having a brilliant time under Manuel Pellegrini, already scoring five goals in just nine games – the same amount of games he played for Chelsea - and the same as Anthony Martial has for Manchester United.
And you better believe that Manchester City fans have had a right laugh at Mourinho for letting De Bryune leave, as the Belgian scored a crucial goal against Sevilla in the Champions League.
So here are some of the best tweets having a cheeky pop about Mourinho's big mistake...
Mourinho watching De Bruyne like... pic.twitter.com/Fx5TsURfhu— Uber Chelsea FC (@UberCheIseaFC) October 21, 2015
Remember when Mourinho said De Bruyne wasn't good enough, but he kept Oscar?— Tom Gordon (@GoonerGordo) October 21, 2015
Every time De Bruyne scores I wanna stab Mourinho in the eye with a spoon— Luscious Lyon (@CliffyBangBang) October 21, 2015
Jose Mourinho after realising he had De Bruyne, didn't rate him so bought Marco Van Ginkel instead pic.twitter.com/999w04GSaP— Mr Middlesbrough ® (@Smogonthetees) October 21, 2015
Leaving Chelsea is the best choice Kevin De Bruyne will ever make in his life— Azam (@AzTalks) October 21, 2015
and there will be gnashing of teeth from Chelsea fans because Debruyne is having a fabulous time with City— King George Armani (@kinjorge) October 21, 2015
Selling Kevin De Bruyne was Jose's biggest mistake— #CHELSEAFAN (@FowlerDarren) October 21, 2015
Mourinho is calling wenger a specialist in failure when he is the one who sold KBD that he wasn't good enough for chelsea.— BALO (@Bruno_Balo) October 21, 2015