Yamaguchi, who failed to qualify for this year's London Olympics, shattered the men's 200-metres breaststroke record in September at a low-key meet in Gifu, central Japan.
But since slicing 0.27 seconds off Hungarian Daniel Gyurta's Olympic gold medal-winning record with a new world best of two minutes 7.01 seconds he has suffered a shocking hangover.
"(Yamaguchi) is better than Kitajima was at his age," Japan's national coach Norimasa Hirai told local media at a short course World Cup event in Tokyo.
"But right now he doesn't have much of anything."
Yamaguchi's world record was 0.50 seconds quicker than the previous national mark held by Kitajima, who won the 100-200 breaststroke double at the 2004 and 2008 Olympics.
However, his new celebrity status is causing Yamaguchi problems with crowds flooding to his local pool on Kagoshima island, southwest Japan, disrupting his training.
"There has been a sharp rise in the number of people using the pool after I broke the world record," Yamaguchi said.
"I haven't been able to find my top speed and my form is still not where it should be. I don't have that explosiveness."
Hirai has warned Yamaguchi to keep his feet on the ground.
"It's what happens when you break the world record and then spend a bit of time back home," said Hirai, who coached Kitajima to Olympic glory.
"If you take your foot off just a little, you'll be nothing but a flash in the pan."
Despite the display of tough love, Hirai was excited by Yamaguchi's potential.
"This is the dawn of a new age for Japanese swimming," he said.
Yamaguchi insisted a crowded local pool would not stop him aiming high at next month's world short course championships in Istanbul.
"I might not be in top form just now but hopefully I can make up for it with my technique," he said. "I think I have a chance to break more world records."
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