"We made hard work of it but the only thing you can do is make chances and we should have been four up at half-time and the game should have been out of sight," he said after the 2-1 win at Pride Park.
"You are sitting there fearing what might happen - and what did happen.
"They haven't created anything at all against us and we have missed chance after chance and we haven't taken them.
"We needed to have taken the chances and that would have been a complete performance because it could have been a rout."