Champions Thailand left for Vietnam on Friday morning to kick off their title defence of the Asean Mitsubishi Electric Cup 2024.
Thailand will face Timor Leste on Sunday in their opening Group A match at Hang Day Stadium.
Sixteen players left for Vietnam on Friday morning and five more players from Port, who played in the AFC Champions League Two on Thursday, will join the squad later.
"We will have only 21 players for the first game against Timor Leste, so we do not have a full squad yet, but the rest of the squad including newcomers have been doing great in training," said Japanese coach Masatada Ishii.
"I hope these new players understand the tactics and that we will have a good first game of the tournament," he added.
Thailand goalkeeper Patiwat Khammai emphasised the importance of the opening game against Timor Leste.
"The game with Timor Leste is very important because it is the first match and getting a good result in the first game will be great for the players' confidence," said Patiwat.
The match against Timor Leste will kick off at 8pm and will be shown live on Thai Rath TV.
After Timor Leste, the War Elephants will play Malaysia at home on Dec 14, visit Singapore on Dec 17 and host Cambodia three days later.
Group B includes Vietnam, Indonesia, Philippines, Myanmar and Laos.