Thailand lost 1-0 to Oman in a warm-up match in the United Arab Emirates on Tuesday night.
Abdulaziz Al-Muqbali scored the winner after 43 minutes in Dubai in Thailand's first of two warm-up games ahead of their World Cup qualifiers in the UAE next month.
The War Elephants will meet Tajikistan in their last warm-up match on Saturday.
Thailand coach Akira Nishino fielded several players, who are expected to play in the World Cup qualifiers, including goalkeeper Siwarak Tedsungnoen, Pathompol Charoenrattanapirom, Sarach Yooyen, Suphan Thongsong, Naruebodin Weerawatnodom and Supachai Chaided.
Thailand had several scoring chances but failed to break Oman's defence.
Playmaker Chanathip Songkrasin, one of Thailand's best players, is expected to join the team on June 2 after his club duty with Japan's Consadole Sapporo.
Meanwhile, striker Teerasil Dangda and defender Theerathon Bunmathan have pulled out of the team.
The War Elephants will play their remaining three Group G matches of Asia's second qualifying stage for the 2022 World Cup against Indonesia on June 3, the UAE on June 7 and Malaysia on June 15.
All matches will be held in Dubai due to travel restrictions as a result of the Covid-19 pandemic.
With the top eight group winners and four best runners-up advancing to the next round, the War Elephants have a slim chance of reaching Asia's third and final qualifying stage.
After five games, Vietnam lead the group with 11 points, followed by Malaysia (nine) and Thailand (eight).
The UAE have six points from four games while Indonesia have no points.