Roberto Mancini's City are unbeaten in seven league matches, winning six and drawing with Fulham at Craven Cottage.
"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."
Ferguson's United currently lead their rivals City by goal difference at the top of the Premier League.