United were twice punished for failing to clear their lines from set pieces as goals from David Silva and Nicolas Otamendi either side of a Marcus Rashford strike saw Pep Guardiola's side open up an 11-point lead over their closest challengers.
City have still dropped just two points all season, and Mourinho acknowledged that his side are unlikely to breach the gap which now separates them from their local rivals.
Asked whether the title race is now over, Mourinho told BBC Sport: "Probably, yes. Manchester City are a very good team and they are protected by the luck, and the gods of football are behind them."
City have now won 14 Premier League matches in a row, equalling the competition record set by Arsenal over two seasons in 2002.