Derby boss Nigel Clough said the quality Leicester City had on the substitutes' bench was the difference between the two sides.
Clough lamented the poor defending that led to them being 2-0 down inside half-an-hour, but he felt his side could have drawn level and even gone on to win the game until the introductions of substitutes David Nugent and Ben Marshall in the second half.
Nugent scored two goals, the first of which was set up by Marshall, as City then eased through to a 4-1 victory.
"The first time Cardiff went in our box the other night they scored and it was the same with Leicester. It's another goal conceded from a set-play," said Clough.
"That's the only thing that's holding us back. The general performance was not too bad for long periods of the game.
"But all that goes out the window if you don't defend properly and we certainly didn't in the first half-an-hour.
"Jimmy O'Connor gave them the second goal but, with all his experience, he shouldn't be giving that away. It was individual errors.
"If you give someone like Leicester the first goal after five minutes, they're going to be all over you.
"They are a powerful attacking side. Then we saw our other side, where as soon as we got a foothold and scored the equaliser whoever got the next goal would go on and win the game. Unfortunately it was them.
"I don't think Leicester took their foot off the pedal, I thought we started to play a bit.
"We got three in midfield and started knocking it around, and they found it difficult to cope with us. We created enough to get the equaliser.
"But then Nugent came off the bench and the quality they could bring off the bench made the difference."