Gianfranco Zola admitted Chelsea were surprised by Derby in their Carabao Cup clash.
Chelsea scraped through to the quarter-finals with a 3-2 win at Stamford Bridge, where they were pushed all the way by Frank Lampard’s side.
Blues assistant boss Zola said: “It was more than a little bit shaky, to be honest, especially towards the end.
“To be fair, a lot is down to them because they really played well. They surprised us. They played with a lot of quality.
“A lot of it was also down to us because the passing wasn’t as sharp as it normally is.
“We certainly need to improve. It’s been positive in some ways but other things weren’t.”