The defending English champions are currently seventh in the Premier League table with nine points from five matches, four points behind leaders Chelsea.
"The reason is that we didn't work well in pre-season," he told Sky Sports News.
"We didn't have all our defenders and this is not normal. We have only conceded two goals more than last year.
"Maybe we have scored less goals than last year after five games, but we had a problem with Sergio [Aguero]."
City are at Fulham in the Premier League on Saturday.