"It is going to be a special and emotional match for me," said the 23-year-old midfielder.
"I was there four years and had a wonderful time. Everything was very positive and I had great team-mates there who I still keep in touch with. Even though we will be rivals, I am going to do the best I can."
Chelsea paid £24million for the man who played a starring role in Spain's European Under-21 Championship triumph this summer.
Mata scored on his debut against Norwich and then against Leverkusen. But if he nets against his former club, he will keep his emotions in check.
He explained: "My priority is to win for Chelsea now. But I will try to be respectful if I do score and not celebrate."