Rancic announced earlier this week that she will undergo a double mastectomy before the end of the year after her October breast cancer diagnosis.
Rivers confessed that she was shocked to learn of the seriousness of Rancic's illness as she is always professional and never complains about her personal struggles while working.
"We want [her] back with breasts, without breasts, we don't care - we want [her]," Rivers explained to The Huffington Post.
"She is the most professional person I have ever met. First of all, she taped Fashion Police last week knowing all this and said nothing. She leaves her problems at the door and comes in and delivers her jokes and says her comments.
"I think she is the bravest women in the world. You have no idea how much we love her."
Rancic was reported to be "in great spirits" ahead of the operation.