The German guided the competition's holders to a fourth Wembley appearance in 12 months by securing a 2-1 victory at the home of the club where he is still revered from his playing days.
But while Rosler may have turned out for both City and Southampton in the top flight, he has bided his time in English football's lower leagues as a manager, firstly with League One club Brentford and now with Championship side Wigan.
He values the job he did helping the Bees bounce back from the dramatic heartache they suffered by losing promotion to Doncaster in the dying embers of last season's campaign, but Sunday's improbable victory at the Etihad Stadium - a repeat of Wigan's FA Cup final win last May - was the sweetest success yet for Rosler.
"Since I've been in the UK I've had very good results with Brentford, in building up a club who will now win promotion and overcoming absolutely difficult scenarios like the Doncaster game of last year then coming through the play-offs," he said.
"A lot of people said the team was dead and buried and then mentally we came through to Wembley.
"I think coming here against one of the best teams in the world and to win, and the way we won - we've not won by just parking the bus and being lucky, we played very good football over long periods. Obviously that is my biggest achievement as a manager."
Rosler enjoys cult-hero status among the City fraternity and was afforded a rousing reception from home and away fans alike before kick-off, a gratuity which touched him.
"This club has been very respectful to me, the reception I got was tremendous," added Rosler.
"I also felt I was very respectful in terms of I didn't celebrate after the win - I shook hands and walked in. Everybody knows the bond and the ties I've had to the club and that will never change."
City made six changes for the match from the line-up which won the Capital One Cup but, even after summoning David Silva, James Milner and Edin Dzeko from the bench, they were unable to force a replay during a riveting second half.
Captain Vincent Kompany was an unused substitute while influential midfielder Fernandinho was left out altogether, yet boss Manuel Pellegrini insisted he was not resting players with Wednesday's Champions League return leg against Barcelona in mind.
"I didn't leave Kompany or Kolarov or Fernandino out because we have to play against Barcelona," he claimed.
"They didn't play because we are making rotation in the cup and Premier League and also because they play 90 minutes for their national squad on Wednesday. So it was important for them to have a rest."