United secured the title on Monday night after they recorded a 4-0 victory over Aston Villa, and Mancini believes that they had all but secured their triumph as many as five games ago.
The 48-year-old told reporters: "It was over three, four, five games ago, not mathematically but it was over. They deserve to win this title, many congratulations to Manchester United for the work they did this season.
"They did better than us, they won a lot of games in a row, it was really difficult. We should be also realistic. If there is one team that did better than you, you can do nothing."
Manchester City are 16 points behind United in second place.