Trotta went against team orders to miss a 93rd-minute penalty on the final day of last season that cost Brentford promotion, but Rosler believes that the on-loan Fulham man should be judged on his upcoming impact at the club.
The 44-year-old told the Daily Star: "It's important that everyone gives Marcello a chance.
"He knows that some people have not forgotten. Of course, people still remember him for that penalty.
"I remember the impact that he made for the club. We should judge him on that."
The 20-year-old netted nine goals in 29 appearances for Brentford last season.