LOS ANGELES (Reuters) - "Good Morning America" anchor Robin Roberts expects to take medical leave for a bone marrow transplant in late August or early September, and several of ABC television's female stars will fill in for her on the morning news program, network executives said on Thursday.

Roberts, 51, revealed on air in June that she had been diagnosed with myelodysplastic syndrome - a rare blood disorder triggered by her treatment for breast cancer five years ago.

Speaking to journalists in Beverly Hills on Thursday via satellite from New York, Roberts said she had "moments of fatigue" while on air, but also has been "energized" by remaining with "Good Morning America" in recent weeks as it beat NBC rival "Today" in the ratings for the first time in 16 years.

"I am getting ready for the bone marrow treatment that will take place," Roberts said. "We are still not sure when that medical leave will start, most likely end August, beginning September."

ABC News executives said there would be no direct replacement for Roberts during an absence likely to last several months. Instead, network stars, including Barbara Walters, Diane Sawyer, Katie Couric and Kelly Ripa have all offered to step in from time to time.

"We have so many people who love Robin so much that everyone is willing to come and join us for a bit," said "Good Morning America" executive producer Tom Cibrowski.

Couric, who is launching her own daytime talk show in September, told journalists on Thursday she was happy to help out in Roberts' absence.

But Couric made clear that she was not considering a return to breakfast TV herself after 15 years as the popular anchor of the "Today" show before leaving that program.

"You need to know when to exit the stage," Couric said of her morning TV career, saying she had great memories of her time on "Today" but was "happy to pass the baton to someone else."