Takuhiro Nakai, 10, and Takefusa Kubo, 13, who have been snapped up by Spanish giants Real Madrid and Barcelona respectively, are YouTube sensations, both are compared to Argentine wizard Lionel Messi by Japanese media.

But Matsui, who played a key role in Japan's run to the last 16 of the World Cup four years ago, warned it could be a long wait before their true potential and value to Japanese football is known.

"I've seen little Kubo and Nakai but it's too early for Japan to pin their hopes on them," the former Le Mans and Saint-Etienne player said in an interview. "We don't know how they will develop physically.

"From 12 to 16, there isn't much difference but 17 to 18, Japanese kids sometimes struggle in comparison to foreign players in terms of size and physique. That will be when we know."

Matsui added: "There's also never any guarantee they will automatically make the jump from youth to the professional ranks."

Japan returned home from the World Cup on Friday after picking up just one point in matches against Ivory Coast, Greece and Colombia, the South Americans hammering the Blue Samurai 4-1 in their final group game.

- Meek exit -

Japan coach Alberto Zaccheroni quit after his side's meek exit, a decision Matsui, who played under the Italian at the 2011 Asian Cup, said was inevitable.

"I don't think the direction Zaccheroni has taken Japan is wrong but results are all that matter," said the 33-year-old. "The World Cup is totally different from friendlies or the Confederations Cup.

"Unfortunately Japan didn't produce at the World Cup. Teams come over to Japan for friendlies largely as tourists but when you see South American teams at the World Cup, their eyes light up. They go to war. We have to learn from this painful experience."

Japan's 23-man squad consisted of 12 Europe-based players, but Matsui suggested the national side would benefit from more players spreading their wings and going abroad.

"There is more physical contact in France and England," he said. "Obviously it can only help to play alongside foreign players in Europe and get that experience."

When Matsui joined Le Mans in 2004, he had no interpreter and was left to fend for himself when his Internet crashed or household appliances went kaput -- frustrating at the time, but experiences which helped his education.

"At training, the only French words I knew were 'Oui' and 'Non' -- I just had to adapt," said Matsui, now with J-League club Jubilo Iwata. "When my fridge broke down I was told to sort it out myself. Shopping, lightbulbs, they were all difficult.

"It was only by my fourth year, when I had enough French to get angry, that I could get things done. It was hard, but you grow out of adversity.

"I don't know how far Kubo or Nakai will go but I do hope they have the drive to prove people wrong. Japan is closing the gap, but we need some super players to come through."