The Citizens remain top of the Premier League, but only on goal difference, after the big-spending outfit fell to a sole Darron Gibson strike.
And the Italian conceded that it was his fault, having failed to prepare the team accordingly ahead of the trip to Goodison Park.
"Probably it's my fault because we didn't prepare very well for this game," Mancini told BBC Sport.
"I thought before the game it was going to be easier but it is never easy.
"The players put everything on the pitch but I made some mistakes during the last three days preparing for this game."
The defeat, coupled with rivals Manchester United beating Stoke City 2-0, saw the Red Devils move level on points with the title favourites.
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