The Spain international was signed by the west London club from Barcelona in 2014. He won the title in his first season with the Blues. However, a disappointing start to the 2015/16 season saw Roman Abramovich show the former Inter Milan and Real Madrid boss the exit door in December 2015.
The 54-year-old returned to management six months later, when he was appointed as the manager at Old Trafford. Mourinho will return to his former club when United take on Antonio Conte's Chelsea at the Bridge on 5 November.
Following Mourinho's departure, fans expressed their frustration towards the players. Former Chelsea striker Diego Costa, along with Eden Hazard and Fabregas were branded as "rats."
Despite these acquisitions, it has not stopped the ex-Arsenal star to still be in touch with Mourinho. Fabregas also suggested that he have had the "best feeling and connection" with United manager after Arsene Wenger.
"I still wonder myself because it was such a shame. You go to bed sometimes and you think of many things and sometimes you cannot sleep," Fabregas told BBC's Premier League Show, as quoted by Goal.com.
"Still sometimes it bothers me because I could look from the outside as a fan and think, 'these guys are not even trying'. But we just forgot to play football.
"You felt that those six months we were nowhere close to what we really were just a few months before. But it happened and I felt bad for it.
"We still talk today. He is probably the manager that I have had the best feeling and connection for. Apart from Arsene [Wenger], the one I have the most contact with is Jose."