Actually, it doesn't matter. Buffon can pass the ball to Dybala in our own box, then Dybala can dribble everybody, get back to our box, then back again to dribble everyone one more time and score, and Buffon gets an assist.
As for Zaza's goal against Sevilla, Dybala didn't get an assist because he wasn't the last one who touched the ball before it reached Zaza. A Sevilla player touched it. For an assist to count, no opposition player should get a touch of the ball until it reaches the goalscorer.