It makes stronger sense if we know that the quality of Italian defenders and of Italy clubs have fallen considerably in the last 2 decades. Despite of the fall of personal defender quality, the general quality of Italian defensive system has remained at good level (lower than before, but it is still good).
People talked too much about how many goals C.Ronaldo and Messi scored in the La Liga but they don't remember that in La Liga, matches tend to be very open, and more goals in all matches.
Once again, when C.Ronaldo transfered from Madrid to Juventus, people said he scored less than previous seasons because he has become older. It is only partially right and the rest part is the Italian defensive system of clubs here affected his total goals.
Messi, will all due respect to his talent, I believe he will score less goals if he play in Serie A, maybe half of what he does in La Liga. Clashes between Barcelona and Italian clubs prove that theory.
Anyone who watched the match between AC Milan and Barcelona in San Siro do remember how Alessandro Nesta neutralized Messi. In that match, Nesta is an old defender who stepped last steps in his career. I can not imagine how Messi can score if he has to face with Nesta, Cannavaro, Maldini, Panucci, Zambrotta in their best phases of their careers.