Chelsea assistant coach Gianfranco Zola admits David Luiz's introduction helped them gain control for their Carabao Cup win over Derby.

Zola concedes Gary Cahill and Andreas Christensen struggled in the first-half.

He explained: "In the first half they were obviously screening and marking Cesc Fabregas a lot so the defenders had more time, and they were not as sharp as they normally are with the ball so that created some situations where we didn't have control of the game. We lost some balls and they did very well to take advantage of our mistakes.

"The second half was better, the passing was better. David Luiz, when he came on, did very well. We were cruising until the last 10 or 15 minutes when it became up and down, a chance for us and a chance for them. That's something we didn't want. Something didn't work out as normal but Derby were an excellent opponent.

"It's fair enough to say players like Cahill play and then don't play for a while, so sometimes they might have a problem. Plus, the way we play means sometimes our defenders are exposed. When I say we didn't defend well I don't mean just the defenders, everyone has to participate to the defensive phase and some things didn't work.

"I felt Gary and Andreas, in the first half, could have done better with the ball because normally the distribution from the back is better than it was tonight, that's the only complaint I can make. But we'll go through it and we'll discuss it."