Messi has previously been criticised for not delivering at major tournaments, but Mourinho is certain that World Cup glory will not change his own perception of the 27-year-old.
"I think Messi is a fantastic player," the Chelsea manager told Yahoo. "He doesn't need to win the World Cup to be considered a fantastic player.
"He doesn't need to win the World Cup to be considered an historical player in this decade, but for me he is not the greatest player of all time."
Messi has scored 42 goals in 92 appearances for Argentina.