Unfortunately for Mandzukic, an assist is counted only if noone touches the ball between the pass and the goal. In both cases, against Barca and Pescara, an opponent got a (slight) touch of the ball before it reached Dybala (Barca) or Higuain (Pescara).
Initially I gave Mandzu the assists, but then I saw that neither UEFA, nor the sites I consult (whoscored, espn) gave Mandzukic the assist.