Koo was a starting midfielder in South Korea's 1-0 loss to Sweden in their Group F opener at Nizhny Novgorod Stadium in Nizhny Novgorod, Russia, on Monday. The FC Augsburg man played 73 minutes and was replaced by Hellas Verona attacker Lee Seung-woo.
With defending champions Germany and Mexico, the team that beat Germany, remaining as their next two opponents, South Korea's chances to the knockout stage appear to be slim, but Koo said the players have not given up their hopes.
"Everyone worked as a team and sacrificed for the team to prepare for this tournament," Koo said before his team's training at Spartak Stadium in Lomonosov, a suburb of Saint Petersburg. "Of course, we didn't give up our hopes and we don't want to give up.
We will try to make turnaround in the match against Mexico."
South Korean players were severely criticized online for their mediocre performance against Sweden. Some angry fans even sent a petition to the presidential office online, asking to axe certain players from the national team.
Koo said he understands that fans' criticism, but because of that, the match against Mexico is more important than what it used to be.
"We really wanted to change people's doubts to happiness, but because of the result wasn't great, I think we have to accept and endure it," he said. "But the tournament is not over, and that's why we need to work hard as a team to make our fans happy."
After the defeat, Koo, who is in his second World Cup, admitted that his team atmosphere went down as many players were disappointed and had regrets.
"Personally, I only looked forward playing for this World Cup in my second half season in the German Bundesliga," he said. "As we failed to get a victory, I do feel some burden, but that's the reason why I wanted to get a victory in the next match."
South Korea used a 4-3-3 formation, with 197-centimeter striker Kim Shin-woo playing upfront, a surprising setup considering that the team mostly used 4-4-2 in recent World Cup tuneups. With a dreadful attacking job, South Korea had none of five shots on target and remained scoreless.
"After we finished playing our friendly against Bolivia, we decided to go with 4-3-3, and move our line back to make our defense solid," he said. "We really thought we could win."
South Korea will next face Mexico at Rostov Arena in Rostov-on-Don on Saturday. Mexico stunned defending champions Germany on Sunday with a 1-0 win.
"Although most of our analyses were focused on Sweden, the players do have data on Mexico on their tablets," he said. "We've been watching that on the bus, or in our own room and the players have been communicating each other."
Koo, 29, said South Korea will be prepared for Mexico's speedy counter-attacks.
"With Chicharito (Javier Hernandez) upfront, they defended well and made quick attacks," he said. "Mexico had a different tactic that they usually used in their World Cup preparations. We need to compare their recent performance with our previous data." (Yonhap)