Carsley, who made four changes to his starting XI, made a triple substitution at half-time, bringing on Genoa right-back Brooke Norton-Cuffy, captain James McAtee and striker Jay Stansfield as he looked to address his side's disappointing opening.
"The first half was nowhere near good enough. The way we started the game, the goals that we conceded were really disappointing," he added.
"It was important they knew that. The changes we made at half-time, [we] could have made any number of changes. I asked for a reaction in the second half and we definitely saw a reaction.
"The performance was not at the level we need. When you think of the reaction in the second half that was more like it. It took us to go 2-0 down to get to that point which is disappointing."
Carsley has not had as much time as he usually would to prepare his squad for the tournament following his stint as interim head coach of the senior team.
He returned to take charge of the under-21s in March and has had to deal with the loss of Liam Delap and Jobe Bellingham after the pair signed for Chelsea and Borussia Dortmund respectively and will feature at the Fifa Club World Cup.
"You know, with the attention that the team get, living up to some expectations, the responsibility of when you put the shirt on, no-one's going to roll over for us," said Carsley.
"We've spoken about that. But the response was very good and I expect us to start like that in the next game, because if we give Spain a two-goal start, it's game over."