Sampaoli's side conceded three goals in the second half - including a strike from outside the box by Croatia captain Luka Modric - as Argentina's hopes of progressing past the group stages were severely weakened.
The manager told reporters after the game that he was "very hopeful", but also admitted that he did not read the game correctly as Argentina's defence looked rocky throughout.
"Everything that happens to the team has to do with the manager, the project did not prosper and it was very difficult for the players to stand out," he said.
"I was very hopeful, I have a lot of pain because of the defeat, and I certainly did not read the game correctly. The key to the defeat was in my decisions."