The Eastlands club are currently at the top of the Premier League table, and have already beaten the Red Devils 6-1 at Old Trafford this season.
Yet ahead of the FA Cup third-round clash between the two sides, the Italian insists their rivals are still the best in the country.
"If you want my opinion, United are the best team in England," Mancini said, according to BBC Sport.
"They have a strong team with a strong mentality and they are at the top."
Mancini's comments come despite the Red Devils having lost their last two games, with another showdown against their arch-rivals next on the agenda.
But the former Inter boss believes that the mentality that has been instilled in the United players by Sir Alex Ferguson is the product of a long-term process.
"We can't be like [United] in two years; they have had this mentality for 25 years," Mancini said.
"We can improve if we continue to win - if we win the championship or other cups. We started it last season. But in two years it is impossible."
Mancini's men beat United in the semi-final of the FA Cup last season and eventually went on to defeat Stoke City in the final and lift the trophy. It was their first piece of silverware in over three decades, and also saw the manager maintain his record of winning a domestic cup competition with every club he has managed.
"It was important to beat them in the semi-final last year and it was important to beat them at Old Trafford [in the Premier League in October]," he added.
"It was not the 6-1; they played for 40 minutes with 10 players, which is very difficult. It was important because it was a good performance."
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