I don't think that's true. Laurence Olivier was originally cast in the part, he even learned to mimic the speech patterns of Italian-American ghetto English quite well to do the role. I don't remember the details of why he was replaced by Brando, but Olivier was Coppola's first choice.
The cast was great with three weak points IMO Talia Shire(Coppola’s sister),Diane Keaton and James Caan.