The 2003-film starred Sanjay Dutt as the lovable goon and gave a new life to the actor's career.
Hirani, however, said the film was originally planned as a TV series and he had approached Madhavan for the titular role.
"Before 'Munna Bhai MBBS' got made into a film, years back I had written it as a series. I wanted to do a serial out of it and that time Maddy (Madhavan) was doing many serials, he was a big star on television and I had approached him and he said 'I am too busy I don't think I can do it'.
"If he had agreed Munna Bhai wouldn't have gotten made as a film," Hirani told reporters.
To which, Madhavan added, "I always knew it (the film) had a better future."
The duo was speaking at the trailer launch of film "Saala Khadoos", which stars the "Tanu Weds Manu Returns" actor and is co-produced by Hirani.
The "3 Idiots" helmer also said that he is working on the next script of Munna Bhai.
"Yes we've been working on a Munna Bhai script. We have 4-5 Munna Bhai scripts written but they were never as good as the earlier two so we kept them aside.
"Now we have an idea which is taking shape. Before that I have a biopic on Sanjay Dutt and then we are working on three-four other scripts," Hirani said.
The bilingual sports drama "Saala Khadoos" is scheduled to release on January 29.
