Muslims celebrated the Eid al-Fitr holiday on Thursday, which marked the end of the fast, but Le Guen was worried that his side would struggle, like they did the last time they travelled to Southeast Asia after Ramadan only to be thrashed 3-0 by underdogs Thailand.
"This period is always a little bit difficult, Ramadan," the former Olympique Lyon coach told reporters in Singapore on Tuesday.
"Two years ago, we played at the end of Ramadan against Thailand and we were punished. We have to remember that lesson."
Le Guen also faces the prospect of trying to lift a deflated side whose dreams of a first World Cup finals appearance ended with a narrow defeat in Jordan in June.
Oman are missing the key trio of goalkeeper Ali Al Habsi, midfielder Ahmed Mubarak and striker Amad Al Hosni for Wednesday's match and Le Guen also has concerns about the artificial turf used at the Jalan Besar Stadium in Singapore.
"I think it will be a tough game and I am respectful of the Singapore team," the Frenchman said.
"I ask my players to be focused on tomorrow's game, because of the period (Ramadan), the quality of the opponents and the context (the artificial pitch) and to be prepared that it will be tough."
Despite the problems, Oman remain heavy favourites to follow up their opening 1-0 win over Syria and make it two out of two in Asian Cup qualifying Group A. The top two teams from the four team group will qualify for the 16-team finals in Australia in 2015.
Singapore were thrashed 4-0 by Jordan in their opening game but new coach Bernd Strange talked up the chances of a shock against his former side.
"It will be very difficult, but it is an open game," the German said.
"Pressure will be on Oman but we will do our best. We will be in good shape tomorrow and will be able to manage something.
I believe if we are motivated and if we stick together as a team, anything is possible," the former Belarus coach said.
The only other Asian Cup qualifier taking place on Wednesday is in Iran where Syria will play a 'home' match against Jordan.
It will be a first competitive match for new Jordan manager Hossam Hassan who took over from Adnan Hamad who stepped down despite leading the West Asians to a first World Cup qualifying play-off against Uzbekistan.
Should Hassan carry on the good work and lead Jordan past Uzbekistan over two legs in September they will take on the fifth-placed side in South American qualifying for a berth at the finals in Brazil next year.