The Red Devils went six points clear at the top of the Premier League table after Robin van Persie scored in injury-time with a deflected free-kick after City fought back from a two-goal deficit.
"We should have been out of sight with the chances we had," said Ferguson.
"But you have to give credit to City.
"They keep going and they score late goals and they have done it against us again. Fortunately we got the last one."