Ronaldo scored his first Champions League goal for the Serie A giants with a tremendous volley in Turin, but they were beaten despite a dominant performance.
Juan Mata’s free-kick and an Alex Sandro own goal saw United claim a surprise 2-1 win to end Juve’s perfect record in Group H.
But Ronaldo insisted there was no reason for Juve to panic after suffering their first defeat in 15 games this season.
“The Champions League is a special competition, where you could be winning, but can’t relax, as anything can happen,” CR7 told Sky Sport Italia.
“We dominated the game for 90 minutes, had so many chances, could’ve killed it off three or four times, but we relaxed and were punished.
“Manchester United did nothing to win the game. You can’t even talk about luck, because you have to find your own luck and in this case we just gifted it to them.
“Now we’ve got to lift our heads, as we played really well and are still top of the group.”
Ronaldo also spoke to Viasport Football and described his remarkable volley.
“I saw the replay in the dressing room. I didn’t realise it was a fantastic goal, but I did a good movement and I kick the ball strong.
“I was happy with the goal, but of course I’m a little bit disappointed because we should win the game with three or four goals. Manchester didn’t do much, they had two chances on set pieces. I know English teams are always looking for set pieces.
“We will learn from our mistakes and we are still top of the group, so it’s all good.”
Ronaldo was asked how far this Juventus team is from going all the way in the Champions League.
“It’s too early, we have to take it step by step. We are top of the group, we’re a good team, but it’s a long way for the final. These people, the club, they dream of winning the Champions League, but we have to take it step by step.
“We have a good team, we have a good Coach, we have good supporters – step by step!”
Juve remain in control of Group H on nine points, sitting ahead of United (seven), Valencia (five) and Young Boys (one).