United thought they had established a match-winning lead through Wayne Rooney's double before City hit back in the second half.
Indeed, once Pablo Zabaleta had levelled after Yaya Toure had halved the deficit, it seemed like the hosts would go on to win, only for Robin van Persie's deflected free-kick to seal the spoils for United, who move six points clear at the top.
"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."