I think TedyKGB wrote probably the best assessment of Dzeko on this forum.
Dzeko can indeed appear to be invisible, useless, slow, idea-less, almost lost for about 60-70 minutes of game (but just appear, not actually be), because then when you just think that he is going to be awful and useless, he starts a sprint, makes passes between the legs of defenders, setting up a quick goal or scores without anyone seeing it happen.
Its almost like Dzeko embraces a style of play that is parallel to Mike Tyson's "pik a boo" style in boxing. Tyson used to cover his face, you think he is scared, or he is defensive, or he is done, and when you relax and he knocks you out. Same thing with Dzeko, he looks slow, he looks useless, defenders get encouraged (because he is under control), and then suddenly, when everything looks really bad, Dzeko moves it into overdrive and BOOM - its a goal.
He does this all the time.
That's why I say - he is not useless - he only appears to be useless, and uses that appearance to his advantage when he needs to surprise you with a score. He does it so well.