Starting Friday, ABC’s “Good Morning America” — which has surged ahead of the “Today” show in recent weeks to become the No. 1 morning television show in America — was without its biggest star, Robin Roberts.

Roberts, who received a diagnosis of a rare bone marrow disorder in April, is about to undergo a bone-marrow transplant that will leave her hospitalized and homebound for four months or more.

The break presents clear challenges, not just for Roberts, who must regain her health, but also for ABC, which earns huge profits from the morning show. It will have to find a way to maintain its nascent winning streak without her.

Roberts signed off from the show on Thursday, a day earlier than expected, because she needed to visit her 88-year-old mother, who is ill, in Pass Christian, Miss. “I love you and I’ll see you soon,” she told viewers, many of whom have gravitated to “Good Morning America” because of her.

There are few if any precedents in the television industry for an extended leave of absence by a host, even on an ensemble show like “Good Morning America.” ABC thus finds itself in an extraordinarily difficult position: It has to keep viewers informed about Roberts’ condition and encourage them to keep watching the program while she is away, but not appear to be exploitative or insensitive.

News coverage and public sympathy for Roberts could help “Good Morning America,” or her absence could lead viewers to try other morning shows. Roberts has been on the program for a decade, longer than any of her co-hosts

In February, back when “Good Morning America” was No. 2, Roberts felt abnormally tired while covering the Academy Awards in Los Angeles. She followed up with doctors and, after some blood tests, underwent her first bone marrow test before a vacation at the end of March.