It will be the first time that Ronaldo faces the Red Devils, with the Portugal international having spent six seasons at United between 2003 and 2009.
And Wenger feels that the magnitude of the occasion could hamper his display when all eyes are on the Spanish capital tonight.
“Ronaldo’s a great player and he can make the difference,” Wenger told the Daily Express.
“But he is up against his former club and that will put him in the eye of a hurricane. We’ll have to see just how he faces up to the return at Old Trafford.
“You could say that Real Madrid have an advantage in that they are not competing for the La Liga title and so they can concentrate on the Champions League. But on the other hand it is logical to assume that United have been resting players, and keeping their best tricks up their sleeves for this tie.”