Le Guen, who left first-choice midfielder Alexandre Song on the bench and also ended goalkeeper Idriss Carlos Kameni's seven-year reign in favour of Hamidou Souleymanou, was left frustrated after a clueless display ended in a damaging defeat and put their tournament progression in the balance.
"We are disappointed with the result but also with our attitude, we were very tense in the first half," Le Guen told reporters. "I expected something else. I felt the players were not playing to the best of their capacity so we were not able to unleash our potential. That is very frustrating."
Cameroon would no doubt have earmarked the game as their most likely three points in the group stage but a first half strike from Keisuke Honda and a gritty defence were enough to nullify the disjointed Africans and seal Japan's first World Cup win on foreign soil.
The Indomitable Lions now face tricky ties against Denmark on June 19 in Pretoria and the Netherlands in Cape Town on June 24.
"We have to win against Denmark, we know that, we will try to recover well to do our best," Le Guen said.
The Frenchman smelt goals against Japan and chose a 4-3-3 formation but their attack, which included captain Samuel Eto'o in an ineffective deeper role, failed to repay their coach's instinct.
Young midfielder Joel Matip, Song's replacement in the side, also failed to make an impact with Cameroon missing the Arsenal man's calming influence on the play.
"Eto'o plays in the position I choose for him and I take the responsibility for that," a downcast Le Guen replied after suggestions his side would have benefited had the Inter Milan man played further forward.
Cameroon defender Sebastien Bassong was already focusing on the next match.
"I'm very disappointed because we expected a win to be honest. It is a very frustrating result for us but we have to look forward and win the next game," he told Reuters.
"It's always hard to start with a defeat but we will try to improve ourselves."
A marked improvement in their performance will be essential against a Denmark side also chasing their first points having lost 2-0 to group favourites the Dutch on Monday.
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