The Blues boss also credited the visitors for the display in winning 4-0 at St Andrews on Boxing Day.
"I didn't think we played with the same fluency and attacking threat as we have done in previous games.
"I actually thought Derby were the most clinical and threatening side that we have played in last eight we've played.
"They showed their quality and class. Their wide players were a constant threat throughout the game.
"We looked nervy and edgy on the ball maybe we conceded early on in the game which took a little bit of confidence away from the team.
"The first goal was a poor one to concede. We had two chances in the middle of the park to win the and we didn't. We ended up backing off and conceded a goal.
"There were moments in the second half when we came out strongly. Looking at the replay of the game I felt it was a stone wall penalty when Wes Thomas was clattered from behind which was not given at 2-0 down.
"Little moments like that might have changed the games and given something to chase. As it was Derby have that clinical edge to them like Martin, Russell and Ibe have in their locker."