The Blues' boss also said his team had done enough not only to have earned a point after a magnificent second-half fight-back, but perhaps should have gone on and won the game in the time that remained.
"It's very hard to lose a derby, but even more so in the last minute so I'm very disappointed," said Mancini. "We dominated the second-half and for this reason, I am really disappointed.
"They scored from their first two goal-scoring opportunities but after that, we dominated the game and fought back really well, but we can't then give a goal away after working so hard.
"I think we made a mistake for the Van Persie free-kick at the end by only putting three men in the wall. I wanted four but I couldn't get the instruction over and I think that cost us the goal.
"We should have defended this free-kick better and we had chances to win this game, but there is nothing we can do about it now."