Ferguson wonders if City can maintain the stability needed to compete consistently.
Asked if the intensity of the current inter-Manchester rivalry will last, he said: "It may last a long time. But these things are also measured by history.
"United bases itself on stability and continuity at the highest level, and has done for decades. City have spent lot of money in recent years. Will it last? Is that a guarantee of excellence? I don't know.
"With what happened in the Premier League last year, the rivalry has gone up by several notches.
"For us, that created a feeling of emptiness, of loss, a real frustration. I had not felt this utter maelstrom of feelings for a long time, probably for 20 years when Liverpool were on top. We had to learn to live with the frustration and, for us, it was unusual. It's not easy.
"But losing that title gave us an extra strength."