IAR is spot on here, as are several others. Most notably, pete is not close to getting this one right.
Looking at this gif again, it's easy to pick apart everyone's role that lead to the Ronaldo score:
Vidic "should not" have gotten beaten to allow the cross.
De Gea "should not" have been glued to the post and thus been out of reach to manage the low cross.
Rafael "should not" have got beaten on the back door.
But in truth Real put together a fantastic play in the box that involved high pace and thread the needle passes. Even Ronaldo's "shot" was crazy stuff, considering the speed at which the ball came, the use of the outside of his right boot and the sliver of angle to get the ball through. That was a "world class" goal every way you look at it. It would be madness to lay that on Rafael.
Had Arsenal been "defending" on the day I don't see how they could have kept Real to under 4 four goals. Real were that good. We just weren't good enough late in the game, unfortunately being down to 10 men.
For what it is worth and I haven't been following the thread to be honest, but on seeing the GIF Rafael is clearly at fault when the cross is coming in his awareness is gone, he is too far off Ronaldo for a start and his body position is all wrong he should be in a position to see the ball and Ronaldo and he's not hence the reason Ronaldo got first run on him and the fact Rafael couldn't block or match him. Anybody saying otherwise is just wrong.