Stuart Pearce's side secured a deserved 1-0 first-leg win in Norwich on Friday night thanks to their seventh clean sheet in nine qualifiers.
And Birmingham stopper Butland believes their solid defence bodes well for Tuesday's second leg in Krusevac.
He said: "We're a goal up and they've not got an away goal so it is a job well done.
"If we don't concede over there we're through. We are very strong as a team defensively."