The actress had signed up to play feminist icon Gloria Steinem in the film opposite Amanda Seyfried in the title role, but she exited the project shortly after her hospitalisation last week (24Jan12).
After her emergency admission to hospital, Moore was taken to a medical facility to treat exhaustion and "improve her overall health".
The movie's producer, Heidi Jo Markel, has now revealed she was stunned to receive a phone call from Moore's representative explaining why the star had to drop out of the part.
She tells People.com, "It was a shock and concern for all of us. We got a call from (Moore's representative) on Tuesday morning and (the rep) said, 'There's been a personal crisis and we need to let you know that.' And we were clueless. We had no idea what it entailed. We just knew she was out, that there was probably a health concern, and to move on.
"It was mostly that we were worried about her health. Movies come and go, but someone's health is the most important. We just want her to be well."
However, Markel is pleased to have hired former Sex and the City star Sarah Jessica Parker as Moore's replacement, adding, "She's incredible. She has incredible moxie (courage) and strength of character and we were just so overjoyed that we could get her and she was interested in doing the part."