Thailand coach Rakphol Sainetngarm said his men must improve their defence after they lost 4-1 to Portugal in their opening match at the Fifa Futsal World Cup in Lithuania on Monday.
In the other Group C game, Morocco hammered the Solomon Islands 6-0.
Thailand will next face Morocco on Thursday, while Portugal take on the Solomon Islands.
Jirawat Sornwichian opened the scoring for the Thais only for the Portuguese to hit back with four unanswered goals.
"We conceded easy goals in the second half and we must tighten our defence against Morocco," Rakphol said.
"The next game is very important for us and our players must focus on the match. We did well against Portugal but lost concentration."
In their sixth appearance in the Futsal World Cup, Thailand are aiming to reach the round of 16 for the third consecutive time.
The Thais began their campaign in defensive mode as Portugal attacked relentlessly in the opening minutes with goalkeeper Katawut Hankampa making fine saves to deny Ricardinho and Bruno Coelho.
But it was the Thais who went ahead in the 16th minute when Kritsada Wongkaeo's lobbed ball found Jirawat, who fired home a volley from close range.
A mistake, however, saw Thailand relinquish the lead when Kritsada fouled Zicky just outside the box, and Ricardinho perfectly set up Coelho to equalise before the half-time break.
Erick unleashed a crackling strike to put Portugal ahead in the 27th minute.
Portugal moved with confidence and doubled their advantage at the half-hour mark when Zicky made it 3-1 before Pany completed the rout.
Suphawut Thueanklang, arguably Thailand's best futsal player of all time, had a quiet game.