Algeria blitzed South Korea in Porto Alegre, scoring three goals in the first half before Hong's men hit back after half-time, although they left their run too late.
Hong was frustrated by his side's defensive concentration at the Estadio Beira-Rio as they made it 10 goals shipped in their past four matches.
Islam Slimani gave Algeria the lead in the 26th minute, holding off both Korean central defenders to score and Rafik Halliche doubled the Africans' advantage two minutes later with a free header from a corner.
It was 3-0 in the 38th minute when Abdelmoumene Djabou took advantage of lax marking to score from inside the area, while Yacine Brahimi scored Algeria's fourth just after the hour mark to put his country 4-1 ahead, thanks to a sharp one-two with Sofiane Feghouli.
"The result speaks for itself, we made mistakes today," Hong said.
"We tried to change matters at half-time but since we conceded too many, it was difficult to turn it around.
"Our defence crumbled in the first half, we weren't well organised."
"We knew they were going to be strong opponents," the 45-year-old coach said.
"Today their team was completely different than the one that played against Belgium.
"I think they were very well organised in defence and our players were subpar.
"We still have a game to play, so I want my players to recover quickly in time for it."
Belgium have already secured a spot in the round of 16 and if they lost to South Korea, Hong's men could advance on goal difference, as long as Algeria fail to defeat Russia in their final game of the group stage.
