Dini confirmed the news on Twitter, saying that he would love to have returned for the prequel, but wasn't asked.
"Kinda hoped I'd be doing another Arkham game in London during the Olympics. Esp, as I had a good source for tx. Ah, well," reads Dini's first tweet.
It was then revealed that Arkham City was Dini's last game, and that he was unaware the prequel would be set in DC's Silver Age.
The unannounced Arkham Asylum prequel is said to focus on the Caped Crusader's first meeting with the Joker.
The "highly stylised" release will also introduce other Justice League heroes such as Superman and Wonder Woman.
It will likely appear in 2014, after next year's DC fighting game Injustice: Gods Among Us.
Rocksteady, meanwhile, is currently focusing on Arkham City - Game of the Year Edition and the Wii U exclusive Armoured Edition.
The Game of the Year Edition features all previous downloadable content, including the 'Catwoman Pack', 'Nightwing Bundle Pack', 'Robin Bundle Pack', 'Challenge Map Pack', 'Arkham City Skins Pack' and 'Harley Quinn's Revenge'.
Batman: Arkham City shipped more than 4.6 million copies during its first week on sale, and won the award for 'Best Action' game at this year's BAFTAs.
Batman: Arkham City - Game of the Year Edition will be released on September 7.
> Feature: 'Batman: Arkham City' - How the Dark Knight went open-world
> Read our review of 'Batman: Arkham City'
Photo gallery - Batman: Arkham City screenshots:
Copyright: Warner Bros.