Nkunku leaped off the bench and scored the sole goal of the game in the 86th minute.
He later said: "The manager said we need quality in the box (when I came on) and we have 10 minutes so he told me to enjoy.
"Good ball from Jadon (Sancho), I felt I had the space behind me and I turn, I was a bit lucky because there were the three defenders but I was able to score."
On his celebration, he said: "For my son. He likes balloons so I just keep on doing it."
On the competition to get in Chelsea's starting XI, Nkunku added: "We are in a big club, there is competition in every position and we just need to work hard in every training session and in every game."