Ronaldo has been in sublime form with Real in 2018, scoring 22 goals in his last 12 consecutive games and Boateng is wary about the 33-year-old ahead of their last-four tie.
Die Roten have already secured the Bundesliga title and remain on course for another incredible treble under the management of Jupp Heynckes - but their star defender concedes Ronaldo and his Los Blancos teammates leave little room for error.
"We can only stop Cristiano Ronaldo as a team," Boateng told German magazine Kicker. "In front of goal, he's like a machine. You can't shut him out completely, he always gets his chances in a game, because of the lines he runs and his excellent timing.
"Real base their game around him and it's important we give him as little room as possible. There is a no more complete striker than Ronaldo. Left foot, right foot, header - he is in perfect control of everything he does.
"I went through only once against him with Bayern, but twice we have gone out. He also has the best team-mates at Real. Shutting him out is only 50 per cent of it, the other players are simply too good for that at this level."