Julio Baptista's winner sees Real declared 'winter champions' as they sit seven points clear of second-placed Barca at the top of the Liga table.
"We are not going to say here that the Liga has been won. In fact, it's the opposite. Seven points are not definitive and now it is important to continue adding," said goalkeeper Casillas.
"We are happy because we have added three points more and against a direct rival on the table. They are three points that we've taken off them, which in the end may also decide the title."
Asked the key to Real's success so far this season, Casillas was adamant: "The truth is that we are working phenomenally well in training and little by little we are recognising the ideas with which Schuster wants to play. I believe that it is a style with which we are all enjoying."