It's not possible to concede that Ronaldo is a better footballer than Messi because he just isn't. You cannot concede that because it palpably isn't the case. That's why there are countless current and former professionals literally queuing up to say that Messi is not only the best player in the world, but the greatest footballer ever. Google it, if somehow you've missed this. If you can't use Google, I'll happily make a list for you!
I have actually seen Ronaldo play in the flesh far more than Messi for United, in fact I'm sure I've seen him play more in the flesh than virtually everyone that has contributed to this thread. There is no doubt in my mind that he is the best player ever to play for Man United, and there is equally no doubt that he is a truly great player.
There is absolutely no problem with conceding that because it's obvious. It's as obvious as the fact that Messi is far superior technically, does more on the field of play, and is just clearly the better footballer. As I said, this is why so many players, former players, pundits, journalists, and objective observers deem Messi to be the greatest player ever, let alone the best player in the world.