South Korea rallied from a goal down to defeat the Czechs 2-1 in Group A, dealing Koubek's men their first loss after a string of six unbeaten matches.
"We really made some mistakes but also made opportunities. Our opponent was very fast," Koubek told reporters. "The better team won.
"We had to fight and we did," he added. "Now we need to move on. We need to be more attacking and creative in the last 30 metres. In the attack we have room for improvement."
Ladislav Krejci put the Czechs ahead with a fine header as they dominated in the air but Hwang In-beom soon equalised and then turned provider for substitute Oh Hyeon-gyu’s winner.
Koubek also said the Czechs needed to see more from key players like striker Patrik Schick and attacking midfielder Pavel Sulc both of whom he substituted in the second half after the two appeared exhausted in the warm temperature.
The Czechs next meet South Africa on June 18.
"We will face a difficult opponent and we have just faced a difficult opponent," Koubek said. "It was our obligation to play until the end so it's clear the players are exhausted."
Despite a first defeat for the 74-year-old since taking the job, Koubek found positives, especially the performance in midfield of Alexandr Sojka who won his first competitive cap.
He also shrugged off the travelling in store for the Czechs who go to Atlanta next week to take on the South Africans while co-hosts and Group A leaders Mexico face South Korea.
"We just have to live with it because that is what others planned for us," said Koubek. "Our logistics team is good but it is not ideal we have to travel that much."
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
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