Nigel Pearson said it was a last-minute decision to name top-scorer David Nugent as a substitute following a slipped disc in his neck.
The Leicester City boss said it was only on Saturday morning that they decided to include Nugent as a substitute after the player came through a training session on Friday.
Even after Nugent rose to head home the first goal, Pearson was still fearful his neck injury would return, but the 11-goal striker came through the game without any reaction.
"We didn't expect him to be right," said Pearson.
"He decided he wanted to try it out on Friday in training. It was very much a case of seeing if there was a reaction on the morning of the game before we made a decision.
"He said he felt okay to play some part. Although he came off the bench, he has made a very good contribution. I am sure he will get the headlines but there were a lot of good performances.
"I was a bit concerned once he scored that that we would have to change again.
"He doesn't like being injured and, at times, we have to protect him. Having said that, it is great to have players who desperately want to play a part even though they have been injured."
Nugent's strikes were vital as the Rams were threatening to equalise after coming back from two goals down after a superb 30 minutes from City, which Pearson described as their best performance of the season.
"I think that is the best we have played in that first 30 minutes," he said.
"It was an outstanding start from us and we deserved to get two goals in that period. I suppose from that period until the end of the first half it was an illustration of how tough this league is.
"I don't think we played badly after that, Derby just controlled the possession more and scored during that period, which gave it a lopsided look to the half.
"I was a bit disappointed that we were only one goal ahead at half-time.
"But we kept our composure well in the second half and, even though they asked a few more questions of us in the second half, we defended well as a team.
