Ronaldo has scored nine goals in 11 games since returning to Manchester United in the summer, including both in the 2-2 draw with Atalanta on Tuesday, and Guardiola has warned his players about the challenge they face on Saturday.
"I didn't see the United match [against Atalanta], I'm going to analyse and to see what they are so I know how good they are," said Guardiola, after watching his side beat Club Brugge 4-1. "We saw it last season. They have one of the best players in history, a guy who can be a scoring machine."
Guardiola insisted in the build up that the clash with Brugge was "more important" than the meeting with United. After Paris Saint-Germain's 2-2 draw with RB Leipzig, City are top of Group A with two games to play.
"People [were upset] for the fact I said today is more important than United and it was," said Guardiola. "Now, the most important game ever is United.
"The second half [against Brugge] was really good. A good game, good victory. We need one point to go through, three points to win the group."
Guardiola has just 48 hours to prepare his players to face United with the derby scheduled for 12.30 UK on Saturday. United will have an extra day after facing Atalanta on Tuesday but Guardiola resisted the temptation to hit out at the fixture list.
"If they want us to play tomorrow, we will play tomorrow," he said. "The schedule is the schedule. We will be at Old Trafford at 12:30 on Saturday."