The 23-year-old winger moved to Stamford Bridge over the summer for £23m after four years with the Spanish side.
"It is going to be a very special and emotional match for me playing so soon after leaving Valencia," said Mata.
"But my priority now is to win for Chelsea. But I will try to be very respectful if I do score and not celebrate.
"The fact of the matter is that I was there for four years and had a wonderful time. Everything was very positive and I had great teammates there with whom I still keep in touch. Even though we will be rivals shortly, I am still going to do the best I can."
The Spanish international set up Fernando Torres, who opened the scoring in Chelsea's 4-1 win against Swansea City on Saturday.