Juan Mata (At Chelsea)
By the time Jose Mourinho arrived at Chelsea in 2013, Juan Mata had already made a good impression on the Blues faithful, winning Chelsea's Player of the Year award two seasons in a row since he arrived.
Mourinho made a sensational return to Stamford Bridge after then-manager Andre Villas-Boas was sacked after only nine months. Thus began Mata's struggles on and off the field under Mourinho.
The Portuguese's style of play saw Mata being pushed out to the wings, a position he struggled to perform in. His inability to adapt to Mourinho's tactics soon saw him off the first team and eventually sold to Manchester United.
The two reunited in 2016 amongst fear of another fallout but the feud seems to have settled down as Mata has featured in games under Mourinho, but is usually still played out of his preferred position.