Ronaldo, 28, joined Real for £80million from United in 2008 and has hit 34 goals already this term.
Wenger said: “Ronaldo is a great player and can make the difference.
“But he is up against his former club — and that will put him in the eye of a hurricane.
“We’ll have to see how he faces up to the return leg. But Madrid against United is as though the final has been brought forward. It is filled with quality.”