David de Gea was called into action on numerous occasions to save United as bottom of the Premier League table side Norwich held their own.
"I haven't seen that many teams play against the top teams like Norwich did so," Rangnick said after the game. "They didn't play like a bottom team.
"In the end we need to be able to compete on that kind of level and be physically ready. There's still room for improvement in this area."
Ronaldo saw a shot beaten away by Tim Krul in the 37th minute and the home goalkeeper also tipped over a Harry Maguire header on the stroke of halftime after Alex Telles hit the bar with a deflected free kick early on.
The Portugal international made no mistake with the spot-kick, smashing it into the left-hand corner.
There was some concern for Victor Lindelof during the match when he went down pointing to his chest a few minutes beforehand. However, he was able to walk off the pitch unassisted.
Maguire said after the match that Lindelof "felt a little bit of discomfort" but that he was fine in the dressing room.
"He can't even remember how it happened," Rangnick said. "I think he had a collision and he had problems for breathing for more than 10 minutes.
"His heart rate was higher than normal. He was shocked and didn't know how to deal with it."
"It's another clean sheet but still a lot of work to do," Rangnick added.
"The second half I was not at all happy with the amount of corners that we gave away, but we got the three points.
"We have conceded the highest amount of goals in the top 10. Now we have two clean sheets which is good but we still need to improve on that away, especially against physical teams or teams that attack high like Norwich did.e"
The result lifted United to fifth place in the standings on 27 points from 16 games, behind fourth-placed West Ham United on goal difference, while Norwich stayed bottom on 10 points.
Information from Reuters contributed to this report.