Koulibaly said he still believed Senegal could advance to the next round despite starting their Group A campaign with a 2-0 loss to the Netherlands on Monday.
"We are still determined because we know that there is something that we can still achieve during this World Cup and we still believe in our chances," he told reporters.
"Our young players will have to show themselves during the next two games. They have to be on their best form to be able to produce in a big match that we have next against Qatar."
The two continental champions meet at the Al Thumama Stadium on Friday, having both lost their opening Group A games. Qatar were beaten 2-0 by Ecuador in the tournament opener and Koulibaly said it was the "little details" that cost Senegal against the Dutch, against whom they conceded two late goals.
"We should have drawn against the Netherlands team. We knew it was going to be decided by the little details. When we lost Cheikhou Kouyate to injury, it was difficult. Losing players like that in the middle of a match is a bit tough.
"However, we tried to play our football. It's a shame we couldn't score that first goal because we had chances. Now we have two finals to play. We are going to give everything against Qatar to be able to play a decider against Ecuador," the Chelsea defender added.
"I think that in defence, we were solid. We showed some good things and the 2-0 scoreline did not reflect the game. But at top-level football it's like that."
