Greece were thrashed 3-0 by Colombia to open their Group C campaign in Brazil, to leave the Euro 2004 winners on the brink of an early exit.
Santos conceded some of his players did not appreciate the occasion, but said he will demand improvement ahead of their games against Ivory Coast and Japan.
"This defeat makes things very difficult for us," Santos said.
"I will talk to my players, we will study our mistakes and we will correct this situation.
"We will present ourselves in a much better state in the coming two games."
"Some of my players have to understand there are more group games coming, this is not over," Santos told reporters.
"Some of them did not seem to realise where we are and what we have to do.
"We knew (Juan) Cuadrado would play like that. There were mistakes made."
"I have to say that the first goal we let in was ridiculous," Katsouranis said.
"He was totally free and that cannot happen like this. The second wasn't great from us either."
