The blot on United's afternoon was provided by De Gea, the goalkeeper who was signed for £18.3m from Atlético Madrid.
He might have come for the David Silva free-kick from which Joleon Lescott glanced City ahead while he was horribly slow to get down and across to Edin Dzeko's 25-yard blast that made the scoreline 2-0.
"City are a big team and the delivery of the ball [for the first goal] was good ... goalkeeper no chance with that," Ferguson said. "The second goal I thought we could have closed them down but the shot swerved a bit and just caught the goalkeeper on the wrong foot. But I think that you have to cope with these things and he's [got] no problem."