The Eastlands outfit suffered defeat at Goodison Park and relinquished its outright lead at the top of the Premier League thanks to a solitary Darron Gibson goal.
The Italian’s side lost its outright lead over title rival Manchester United as a result of the loss, and only has a predominant goal-difference as a buffer between the two teams.
Darron Gibson scored the solitary goal of the game after a long-range effort swerved into the top corner of Joe Hart’s net, and the visitors failed to capitalize on their dominant possession.
Mancini conceded that he failed to prepare the team for the match and thought the game would be easier for his superstars.
“[It’s a] bad night for me,” he told reporters. “We didn’t deserve to lose, but I think there was some of my mistake in this game. We didn’t prepare very well for this game and we had a problem.
“It’s important for me to know what this mistake is. I didn’t prepare well because I thought maybe this game was easier and you know here [Goodison Park] is never easy, never.
“I didn’t prepare well with the training in the last three or four days.”
Asked if his players put in the same effort as the host, he said: “Yes, the players put everything on the pitch, but I made some mistakes to prepare them in last three days to prepare them for this game.
“[I thought it would be easier] because I thought we played well against Tottenham, we won the last three games in the Premier League and I thought Everton were without three or four important players.”
Regardless of the loss, the former Inter coach remained confident that his side have the quality and the time to stay ahead of the chasing pack.
He said: “It is not important. We are on the top, we have 15 games [left]. It is important that next Saturday we win, however, we have time.”
Mancini’s opposite number and the happier manager, David Moyes, expressed his pride for his players to come through a tough tie and get the result.
“I have [a lot of pride from the performance]. That’s as good at Everton as it has been for a long time. The players we were missing says a lot about the club and how they always seem to come and do it, the players have been great tonight," he told reporters.
“We might have played smoother football and played better, but a side we were playing against tonight I think the players deserve a lot of credit. Manchester City are a magnificent team, we had to defend for long periods, especially in the second-half.
“We limited them to not too many chances, but they’ve always got the threat with the likes of [David] Silva making a through pass, or with [Samir] Nasri with the ability he’s got. It was a great effort by the players and they sowed great desire to get a result.”
American Landon Donovan provided the game-winning assist, his fifth assist in seven Everton games since joining on loan this month.