The young War Elephants began their campaign with a 2-0 loss to Indonesia in Group B on Tuesday (Nov 26).
On paper, the three-time defending champions should have few problems against Brunei, who were hammered 6-0 by Vietnam in their first game of the U23 tournament.
However, Akira Nishino's men need a big win over the minnows as goal difference could be crucial.
After the opening games, Vietnam and Indonesia have three points each, Laos and Singapore one point each, and Thailand and Brunei are still looking for their first point.
The Thai players did not train on Wednesday and only had a light workout in the gym.
Although Thailand got off to a losing start, they remain confident that they will be able to bounce back and reach the knockout stages.
Forward Supachok Sarachat lamented Thailand made too many mistakes in the game against Indonesia.
"We should have done better. We had several chances but failed to score and were punished by Indonesia," he said.
"We have to look forward and start anew. From now we have to get three points in every game and I believe we can do it."
The Buriram United star is one of Thailand's key players who have played for the national senior team.
But Nishino did not field him in the starting line-up against Indonesia and he came on as a second-half substitute.
Supachok defended the Japanese coach's decision, saying he was still tired from playing for the national senior team in World Cup qualifiers.
"I felt tired and the coach wanted me to rest," he said. "Now I am ready to play."
Thailand defender Saringkan Promsupa said the morale in the team was still high.
"We have to forget the previous match. We have learned a lesson. Although we started with a defeat, I believe that we can win all remaining games and become champions," he said.
"Every game is difficult but we must believe in ourselves. We must believe that we can do it. I think we will get better and better."
Group A comprises the Philippines, Malaysia, Myanmar, Cambodia and Timor Leste.