City lost 4-2 to Aston Villa in the third round of the Capital One Cup last night to make it 14 goals conceded in just seven matches in all competitions so far this season.
Without a clean sheet, Toure believes that there needs to be a massive improvement in the City defence if they want to reach the same heights as last term.
"It was a disappointing night," said Toure.
"The team didn't play well. We conceded four goals, which is not good, and we gave some cheap goals to Villa as well.
"It concerns us because a team like Man City cannot expect to concede four goals at home.
"Since the season started we have not had a clean sheet and that is very important when you want to be a top club, you have to defend better as a team.
"As a defender and as a group we have to work on it because mistakes were done by a few players and when you win it is everybody so when you lose it is everybody."