The Premier League title holders' slim hopes of qualification for the Champions League knockout stages rest on a result on Wednesday but the Portuguese forward, whom the City boss says should win the Ballon d'Or, will pose a threat.
Asked by reporters how he plans to stop Ronaldo, Mancini jested: "We'll call the police.
"He's a top player and he deserves to win the Ballon d'Or. He scored so many goals last year and that helped them to finish above Barcelona [in La Liga].
"I hope he can have a bad night on Wednesday. When you come up against a player who can score one or two goals every game it is difficult."
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The match represents the first time that Ronaldo will play in Manchester since leaving City's rivals, United, for Madrid in 2009, a significance not lost on Mancini.
"For him it will be, yes, but also for them it's an important game," the Italian noted. "They still have to qualify.
"And for Ronaldo, he's coming back to Manchester."
Mancini also added a word for Edin Dzeko, whom he left on the bench against Aston Villa on Saturday despite several recent goalscoring cameos as a substitute.
"He is an important striker," said the 47-year-old. "He deserves to play from the start, like Sergio [Aguero], Carlos [Tevez] and Mario [Balotelli]."