However, United could have been forced into rueing their missed chances as the home side came out after the break looking a completely different outfit and only for Ronaldo's goal 14 minute's from time may well have taken at least a point from the game.
"In the first 20 minutes we should have been four or five up and that would have made it much, much easier for us," Fergie said after the 1-0 win at Pride Park.
"You have to admire the second half from Derby. They made a real battle of it, but the name of the game is to win and we have to thank Ronaldo again.
"It was probably his hardest chance of the match because he had a lot of tremendous chances throughout the game.
"Everybody trusts him now and what you have to admire about the boy is that his courage is beyond question. I don't know how many fouls he has on him every game.
"People want to see footballers like him and I think we have to give him encouragement and we do that. We tell him to keep doing what he is good at, not to stop."