For the second game running errors at the back left the Three Lions disappointed, with them now needing a draw against Colombia to take their place in the third-placed play-off
Cauley Woodrow's early goal was cancelled out by Lee Changmin's equaliser for South Korea, meaning Southgate's side must now draw against Colombia in their final group game to reach a play-off for third place.
Centre-backs Michael Keane and Ben Gibson were guilty of backing off Lee against the Koreans, with those mistakes coming after errors from Jack Butland and Liam Moore in defeat to Brazil.
And though England are missing three guaranteed starters - Jon Flanagan, John Stones and Luke Shaw - due to their involvement in the senior World Cup squad, Southgate believes his defenders must learn their lessons quickly.
"We gave a very soft goal away," he told BT Sport.
"We won't win matches if we concede the types of goals that we're conceding at the moment. Players have to go through this and that's one of the reasons we've come here.
"We've wanted to rotate the squad and give people opportunities, and we'll find out a lot about them over the next few days.
"I'm very happy with the way we played in the second half, but it's very simple: if you give poor goals away, which we have in three of our last four matches, it gives you a difficult task."
Southgate made four changes to his side after the defeat to Brazil, and though all didn't pay off, the decision to bring Woodrow into the side did prove the correct one.
The 19-year-old almost had a hat-trick as he had a goal ruled out for offside late on as well as a header miraculously saved by Kim Dong Jun in stoppage time, and the former Middlesbrough boss was impressed by the forward.
"It was a really good finish," he added. "He's trained really well with us and deserved his opportunity.
"He's a good finisher, he gets hold of the ball well and I'm really pleased for him that he got his goal as well."