The Portuguese scored his first European goal for his new club after firing a volley past David De Gea to put Juve ahead in the second half.
But Juan Mata’s free-kick levelled the scores before Alex Sandro’s own goal gave the Red Devils an unlikely 2-1 win.
And Ronaldo insists that Juve deserved all three points after missing several good chances to put the game to bed.
“I was happy with the goal but we are disappointed because we should win the game easily, by three or four goals,” he told Viasport.
“Manchester didn’t do much, they created two chances from set pieces. English teams are always looking for set pieces.
“But we will learn from our mistakes and we are still top of the group and we are good.”
In truth, Juventus could have had the game wrapped up before then with Sami Khedira and Paulo Dybala hitting the woodwork.
“The Champions League is a special competition, where you could be winning, but can’t relax, as anything can happen,” Ronaldo added.