The Three Lions suffered their first qualifying defeat in 10 years when they were beaten 2-1 by the Czech Republic on Friday.
Southgate admitted it was ‘the poorest’ England had played ‘for a long time’ as goals from Jakub Brabec and Zdenek Ondrasek cancelled out Harry Kane’s fourth-minute penalty.
England lead second-placed Czech Republic on goal difference in Group A – and Southgate is looking for a response from his team against Bulgaria.
He said: “I don’t think defeats harm teams. If the reaction is good, then what you learn from defeats, what you gain in resilience from defeats, are an important part of a player’s career and a person’s life.
“Maybe we were all focusing a lot on next summer, talking a lot about next summer, answering questions about a home tournament and I’ve always maintained that the first thing is we have to qualify.
“We haven’t secured that qualification yet, so our focus tomorrow is to get the performance and the result that we need.”