City squandered the chance to open up a three-point lead at the top of the Barclays Premier League as they conceded at injury-time goal at the Stadium of Light on Sunday.
The defeat compounded the frustration felt after a goalless draw at West Brom on Boxing Day and there is little time to dwell as Liverpool visit the Etihad Stadium on Tuesday.
Mancini said: "We missed a big opportunity, a big chance to go three points up on (Manchester) United. I thought we threw three important points out of the window.
"It was important but now it is finished and we can do nothing. But at the same time, when we arrive back in Manchester, we need to think positively for the next game and for the future.
"We play again quickly and we should have good concentration for the next game. We have 45 points and we have scored 53 goals, and I think we need another 45 points in the second half."