The Red Devils have got off to a shaky start in the Premier League this season, with three defeats from their first nine games leaving them in eighth place in the table.
Despite admitting that it will be difficult for City to be crowned champions, Toure believes that United will find it even harder to successfully defend their title.
"I must admit that we have a tough battle ahead to reclaim the Premier League crown," the Ivorian told Goal.com.
"We must work extra hard as a team to compete with Arsenal, Liverpool, Chelsea and Manchester United.
"United will find it hard to defend the title since they have a new manager, David Moyes, and are really struggling."
City are currently one place above their local rivals in the table following Sunday's 2-1 defeat at Chelsea.