Harper, 73, revealed this week she could have just months to live after doctors diagnosed her with a rare form of the disease in January (13).
The actress plans to undergo chemotherapy but appears resigned to her possible fate, telling People magazine, "I have a fighting chance until I'm gone, (but) I'm well passed my expiration date already."
However, her The Mary Tyler Moore Show co-star Asner is convinced Harper can prove doctors wrong.
He tells the New York Post, "I have every confidence in the world that she will shock the hell out of us and survive - to keep functioning as the great talent and human that she is."
Veteran actress Betty White has also spoken out about her friend's diagnosis, telling the publication, "Here I go wheezing on at 91 years old, and this beautiful, beautiful lady is facing something far too soon. We all love her so much, and we are going to make the most of every second we can."