The two clubs meet later this month in the first derby of the season.
"I've said all along that would happen," Ferguson told Inside United.
"They are a good team and they were always going to be there or thereabouts.
"Sergio Aguero was bound to do well, coming into the Premier League, because he is such a goal threat around the penalty box. David Silva is also a very good player.
"We just have to make sure we are ready for City when we meet them."
Previously, Ferguson has cheekily suggested Liverpool were United's true derby rivals.
Sheikh Mansour's £1billion investment in City has changed all that, with the plethora of stars working under the guidance of Roberto Mancini bringing an extra dimension to the fixture.
"All derbies are big now, that is the nature of the landscape," said Ferguson.
"The impetus City now have - they have a huge squad of players and the financial power to attract some of the best footballers in the world - actually changes the focus of the derby these days.
"The talking point isn't so much the actual game but the dynamics of how Manchester City have changed football and what we do to compete with that."