The 29-year-old Portuguese forward is the current Ballon d'Or holder and has scored 21 goals in 14 games so far this season for Los Merengues.
Former Real boss Capello is quoted by AS as saying: "Cristiano Ronaldo and Messi are two extraordinary players but they are different.
"Cristiano is a powerful player, with very good technique, but he does not possess the pure technical skill of Messi. Leo Messi is a very difficult player to play against."
Messi has won the Ballon D'Or four times and reached the final of the 2014 World Cup with Argentina.