LONDON. Roberto Mancini has hailed the retiring Sir Alex Ferguson as the greatest manager of his generation and said his achievement of coaching Manchester United for almost 27 years will never be repeated.The Manchester City manager believes his United counterpart is a complete one-off but does not feel the Premier League champions will fall into decline after Ferguson is replaced by David Moyes.Ferguson, who took over from Ron Atkinson in November 1986, won 13 Premier League titles, two Champions Leagues, five FA Cups and four League Cups in his time at Old Trafford.And Mancini says it is the former Aberdeen manager’s longevity that makes him unique."Everyone can have his opinion but [when] 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,” he said.Mancini stopped short of proclaiming Ferguson the finest manager in football’s history but believes the Scot is the outstanding coach of his era."In Europe, there are other big managers,” he said. "It is difficult to say Sir Alex is the best [ever] but he is the best manager in the last 27 years.”Mancini has been Ferguson’s immediate rival during his three-and-a-half years as City manager and won the 2012 Premier League title at United’s expense.He also emerged victorious from his last two Manchester derbies at Old Trafford, including the 6-1 win in October 2011, which was Ferguson’s heaviest defeat as a manager."For me, it has been a great honour and pleasure to compete against Sir Alex for three years,” Mancini said. "It was a great honour to beat him at Old Trafford and I wish him good luck for his future. He took the decision [to retire] and we should respect his decision.”Mancini believes United have the infrastructure and the power to flourish without Ferguson."At a top club, a big club like United, you can change the manager,” he said. "Sir Alex built this team but it is a big club with a big history and I don’t think it will change everything.”