Mathieu left Barca over the summer for Sporting on a free transfer.
Asked what he'll do if he scores, the Frenchman told Marca: "I have a lot of respect for my former teammates and the fans.
"If I celebrate a goal, it will be for (president Josep Maria) Bartomeu and (sports director) Robert Fernandez, because I think the way they treated me was very difficult for me."
Mathieu added: "It was incredible for me to share the dressing room with players like that. Friends. Friend is a very big word."