It was a case of persistence is a virtue from the Portuguese star as chances came and went before he finally broke the brave Rams' resistence in the 76th minute.
It was also a case of Ronaldo picking himself up after last week's FA Cup defeat to Portsmouth at Old Trafford.
The 23-year-old expressed his frustration after the Portsmouth game as he felt he was afforded little protection by the match officials.
At Pride Park, Ronaldo was bullish as he kept plugging away until he eventually grabbed the winner after earlier striking the post, whilst also seeing Roy Carroll pull off a number of fine saves.
"In the first 20 minutes we should have been four or five up, and that would have made it much, much easier for us," said United boss Sir Alex Ferguson.
"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 a fantastic goal because it wasn't easy to take
"The ball bounced up off the turf and he had to get up and over it and make sure he kept it down. It was a really good finish.
"It was probably his hardest chance of the match because he had a lot of tremendous chances throughout the game."
Ferguson added: "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. Last week there were three in the first seven minutes, and seven in total, yet no-one was booked.
"So he has the courage to get up and get on with it.
"He made a comment last week that he is getting a bit frustrated with it all, but he has to trust the 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. That is what we would hate to see."
But United's victory means they remain firmly on course for back-to-back titles, with Ferguson adding: "It's three points and we're pleased with that.
"We've a home game with Bolton on Wednesday, and hopefully we can win that as well.
"I think the league now is bubbling up very nicely. It should be a fantastic finish to the season."
With regard to the imperious Ronaldo, Rams boss Paul Jewell joked: "If he was good looking, he would probably have everything!
"But he's a brilliant footballer, surrounded by brilliant footballers."