The Blues were trailing at the break, but the second-half was a different story as Clark's changes led to a brace from Thomas and a long-awaited first goal of the season for Redmond at St Andrew's.
"Redmond has waited a long time for that first goal of the season and it was a fantastic strike," Clark said.
"I have said all along that I have faith in Thomas," Clark added. "There is a lot of pressure on him and he showed me against Derby the form he displayed at Bournemouth, Blackpool and Portsmouth.
"Even though there was not too much difference in the first half I thought that Derby were gaining momentum by each minute and the game was there to be taken if we wanted it.
"Derby have a lot of energy and a great work ethic and we had to match that if we were to get anything from the game.
"They were more energetic than us in the first half and I decided to get the two lads on with a bit of power and pace. I am delighted for both of them.
"The result was that we played with a lot more power, pace and energy and we did to Derby in the second half what they did to us to us in the first half."
Clark added: "We played more on the front foot in the second half and I thought young Shane Ferguson was outstanding from start to finish.
"After we played Hull a couple of weeks ago Steve Bruce said it was about who makes the least mistakes and I think that is absolutely spot on."