"We deserved to win this game, it's incredible, but this is football," he added.
"Last year here was the same situation, I don't know how many chances we had, and we lose the game in the end on a penalty. And [this time] apparently the goal was offside.
"A team like us, we can't concede three of four counter-attacks at the end of the game. Sometimes when you are on the pitch you need to understand that there are some games when you can't score.
"At that moment, you should understand 'OK we stay' and one point is better than nil."
Defeat for City is their second in the league this season, and means they have not won away in four games. It also marks the second consecutive game in which they have not scored after
a goalless draw with West Brom on Boxing Day
Mancini explained: "We played two teams defending with all their players in the box, but we are missing a lot of chances."
City now face Liverpool at home on Tuesday, 48 hours after playing Sunderland, and face Manchester United in the FA Cup next Sunday.
said he was delighted with his side's performance.
The result earned O'Neill a third win in five league matches and he said: "It's been an incredible weekend for teams and we are delighted with three points.
"It was just a remarkable performance by the team.
"The roar for the goal was deafening, and I was delighted to hear the roar for the final whistle as well.
"It was a fantastic effort, really, really fantastic."
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