The 48-year-old has locked horns with the Scot since he took over at the Etihad Stadium and, while their relationship often became fractious, the Italian respects his adversary
The 48-year-old described the Manchester United boss as the best manager of his generation and revealed how he relished going up against the legendary Scot in their Manchester derby rivalry.
"For me, it has been a great honour and pleasure to compete against Sir Alex for three years," he told reporters.
"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."
When asked whether Ferguson's retirement will tip the title balance in City's favour in the future, Mancini replied: "No. I think that when there is a top club like United, you can change the manager, an important manager like Sir Alex who built this team, but I think the club will stay a big club."The Italian recalls his two wins at Old Trafford as special moments in his managerial career and also claimed he sent a message to Sir Alex following the 71-year-old's retirement.
"It was a great honour to beat him at Old Trafford. I wish him good luck for the future.
"For me it was fantastic to compete against him in every derby because he was a top manager. I think we should respect his decision."
As for life after Ferguson, Mancini believes the Red Devils will continue to go from strength to strength.
"It has a big history. I don't think that will change."