Man Utd boss Ferguson: Man City now my biggest challenge
Ferguson said: "Challenges are what we are made of. We face them head-on, we don't run from them.
"It's fantastic. No dull moments.
"City are our biggest threat now, there's no question about that and we are their biggest threat.
"The great thing about football is that things can change and their fortunes changed when Sheikh Mansour took over.
"I knew the minute that happened it was going to be a different ball game."