The Bees had the chance to seal their place in the Championship when they were awarded a last-minute penalty against Doncaster, but Marcello Trotta crashed it against the crossbar and Rovers broke down the other end to score.
That meant Doncaster went up as champions and Brentford were consigned to the play-offs, where they face Swindon.
But Rosler is determined not to throw an entire campaign's hard work away.
"Our season is not over," he said at the club's end-of-season awards night.
"I said to the players before the game that I could not have wished for a better group for my first job in England.
"They have really helped me. I enjoy working with them and the belief I have in them is endless.
"I know how far they have come in two years. The sky is the limit. We had a setback but we will be there. We have bounced back before and we will do it again."