Mourinho signed the midfielder from Barcelona in 2014 and he played under him for a season and a half before the Portuguese was eventually sacked by Chelsea.
Fabregas had always played for Mourinho’s rivals and he admits it took until the two worked together before he truly appreciated him.
"Jose Mourinho? He was my enemy when he was in Madrid or Chelsea,” he told Vanity Fair.
"We were in competitions and we wanted to win against the other. Then I became his player at Chelsea and I was thoroughly behind him. This is football today.
"I learned a lot from him and I have very positive memories with him that I would keep for life."
Chelsea won the Premier League during Fabregas’ first season at Stamford Bridge and the Spaniard added a second to his trophy cabinet last season under Antonio Conte.
Now at Manchester United, Mourinho is aiming for his fourth league title in English football.
The two sides come face-to-face at Stamford Bridge on November 5.
Following this week’s Champions League fixtures, both teams return to league action this weekend, as Chelsea host Watford and United visit Huddersfield.
