The gap between the two Manchester giants remains at nine points following City's 3-1 defeat at Southampton, where Mancini blasted his side's performance as "worse than poor".
United can widen the gap to 12 points with a victory over Everton on Sunday and Mancini believes that margin could be difficult to close in the final 12 games of the season.
When asked if he was conceding the title to United Mancini said: "Maybe. 12 points is too much at this moment and we have not played well in the last two games.
"This is strange because we played the whole of January really well, I don't know why we played so badly.
"Southampton are playing well and it is difficult to score against them - I don't think a team like us can do a performance like this."