Capped 51 times by Portugal and now an assistant coach for the national team, Sousa says Nani has enormous potential.
"I think that in the next five or 10 years, he and Cristiano Ronaldo can become the best players in the world."
Cristiano can play anywhere and is very strong on both feet whereas Nani is basically a left-sided player.
"Nani is still young but has progressed very quickly. He will face strong competition at Manchester which should help him progress," said Sousa.
Sousa admitted that Nani is not yet the finished product: "He does need some time and will need to grow up in the correct way.
"He has Sir Alex Ferguson who is a man I love because of the way he brings players on.
"Nani has incredible talent and Sir Alex loves to work with such talented players and is brilliant at creating big players."