Rosler says it is obvious Dzeko didn't get along with former City boss Roberto Mancini.
The Brentford manager told the Manchester Evening News: "Last season, I guess and judging by his body language as well, the chemistry between him and the manager (Roberto Mancini) was not always of the optimum.
"I am sure that would have affected his confidence in his general play.
"Edin, I think, has already said this season in an interview that he feels like a new player, he feels confident and he feels he has the support of the new manager.
"Strikers need that confidence from a manager.
"If they go for a certain spell without scoring any goals, they still need to be playing and still be the focal point of what the team is trying to do.
"When Edin gets that confidence I think he can deliver for City what is needed."