It's possible that the lack of defensemen who can carry the puck drags him down but I really don't see Drouin making it as a center. Sergachev might have given the other centers some support while also providing the cap space to bring back Radulov and Markov. Adding Drouin would have been a bad move even if they had traded less for him. He provides another small fast forward to go with all the other ones the habs already have. He may be a slight improvement over some other wingers but not enough for the money he's paid. He's unlikely to have a long career as he is small and his game is based on speed. Sergachev will probably be playing solid hockey long after Drouin retires, is likely to outscore him over his career, and will certainly help his team outscore others more effectively.
A small forward who scores on the power play and neglects defense is a net negative. NOTE: this does not describe Byron or Gallagher, though it could apply to Galchenyuk.