Mourinho is close to the Manchester United manager.
Mourinho told The Offisde Rule podcast: "We will all miss him and, for me, it will be difficult to go to Old Trafford, especially for the first time, to play against Manchester United without such a figure in the opposite dugout.
"He is unique and probably Manchester United are a unique club because they gave the manager the conditions to do the job he did.
"Everybody knows that the beginning was not easy for him. Everybody knows that it took him three or four years to win the first piece of silverware and United gave him the conditions to do an incredible job. After that he was unique, absolutely unique.
"The number of trophies he won for Manchester United in English football is absolutely amazing and to manage for more than 20 years is incredible."