Mancini feels honoured to have competed against Ferguson, who the Italian believes to be the greatest of his generation.
Mancini said: "For me, it has been a great honour and pleasure to compete against Sir Alex for three years.
"It was a great honour to beat him in Old Trafford and I wish him good luck for his future. I don't know for which reasons (he has retired).
"Everyone can have his opinion but one manager who stays for 27 years in the same club and won every trophy for 27 years - it is an incredible situation.
"I don't think there will be another manager like him.
"It is difficult to say Sir Alex is the best (ever) but he is the best in the last 27 years."