It's almost too much for anyone to bear ... Now we know why "Good Morning America" anchor Robin Roberts left a day before her scheduled medical leave -- her mother died Thursday.

Roberts -- who will be taking several months off to undergo treatment -- including a bone marrow transplant -- for a rare blood disorder -- flew to Mississippi Thursday to see her 88-year-old mother, Lucimarian Tolliver Roberts, before she passed away.

Roberts -- who battled breast cancer and developed the blood disorder as a result of chemo -- will be getting the life-saving bone marrow from her sister, Sally-Ann. The procedure is scheduled for next week.

Robin Roberts just announced ... she will begin her medical leave from "Good Morning America" TODAY ... one day ahead of schedule ... after "some things happened at home."

Roberts -- who was recently diagnosed with MDS -- had originally planned to stay through tomorrow before leaving the show to prepare for her bone marrow transplant.

But this morning, an emotional Roberts explained, "I had planned to let tomorrow be the last day ... and then some things happened at home and I wanna take care of that before I begin my treatment next week."

Country star Martina McBride performed a song for Robin ... and Roberts, with a smile on her face, tearfully sang along.

Roberts vows to return to work following the procedure.

Robin Roberts was forced to back out of the first interview with Chad Johnson's estranged wife Evelyn Lozada since he allegedly attacked her earlier this month -- and TMZ has learned, it's due to Robin's health crisis.

Sources tell us ... Evelyn agreed to sit down with Robin on "Good Morning America" to discuss the Aug. 11 domestic violence incident -- because she feels Roberts has tremendous integrity as a reporter.

The interview was set for this week, but hit a snag when Lozada asked Robin to tape the segment at her hotel ... since Evelyn wanted a private setting for such a serious conversation.

We're told Roberts couldn't make that happen because she would have to miss a treatment session for her blood disorder ... myelodysplastic syndrome (MDS).

As we reported, Roberts is scheduled to start a leave of absence this Friday to receive a bone marrow transplant.

We're told Amy Robach is stepping in for Robin, but Evelyn's more than happy with the replacement.

Robin Roberts will begin her medical leave from "Good Morning America" on Friday ... so she can undergo bone marrow transplant surgery.

Earlier this year, Roberts announced she was diagnosed with myelodysplastic syndrome (MDS) ...  a disease that attacks the blood and bone marrow.

Roberts says her sister is the bone marrow donor ... explaining, "[She] is a perfect match and she’s been spending a lot of time with me.  She’s been taking shots to help her prepare."

Roberts says the procedure is scheduled for Tuesday ... and says she plans to return to work following her recovery.

"I know I’m going to have a job when I come back."

As TMZ previously reported, "GMA" is planning to use a roster of star-studded subs to fill in for Roberts during her absence.

"Good Morning America" won't be left high and dry when Robin Roberts goes on medical leave in a couple months -- TMZ has learned, producers are arranging a star-studded list of substitutes to sit in for her.

As we reported, Roberts was diagnosed in April with myelodysplastic syndrome -- a serious blood and bone marrow disease -- and has already begun chemotherapy treatment ... before her big bone marrow transplant in a few months.

As for who made the temp list -- we're told producers will approach Diane Sawyer, Charlie Gibson, Barbara Walters, Chris Cuomo, Connie Chung, Joan Lunden and others. We're told each substitute will sit in Robin's chair for a full week, and each Friday the next star sub will be announced.

Given the nature of the operation, it could take Roberts 4 months to recover -- so the list will be long.

But here's the good news. Robin, we're told, is in good physical shape and the condition was detected very early, so the prognosis is "very good" and she could be back much sooner than the 4 month estimate.

"Good Morning America" anchor Robin Roberts has announced she was just diagnosed with a serious blood and bone marrow disease ... but says she's determined to beat it.

51-year-old Roberts just released a statement saying, "Today, I want to let you know that I’ve been diagnosed with MDS or myelodysplastic syndrome.  It's a disease of the blood and bone marrow and was once known as preleukemia."

She explains, "As many of you know, 5 years ago I beat breast cancer.  I’ve always been a fighter, and with all of your prayers and support, a winner. Sometimes the treatment for cancer can cause other serious medical problems."

"My doctors tell me I'm going to beat this -- and I know it's true."

Roberts says she will begin pre-treatment starting today -- chemotherapy in advance of a bone marrow transplant later this year.

Roberts adds, "Bottom line: I’ve been living with this diagnosis for awhile and will continue to anchor GMA.  I love what I do and the people with whom I do it.  Along with my faith, family and friends, all of you at ABC News give me the motivation and energy to face this challenge."

For the first time in 16 years, ABC's "Good Morning America" edged out NBC's "Today" in the morning show ratings last week -- and this morning, ABC News rubbed it in NBC's face ... throwing a champagne-popping victory party.

All the big swinging chicks at ABC were present ... including Robin Roberts, Diane Sawyer, and Lara Spencer -- in addition to ABC News President Ben Sherwood, Senior EP Tom Cibrowski, former "GMA" host Charlie Gibson, George Stephanopoulos, Sam Champion ... pretty much the whole crew.

We're told Robin Roberts toasted the entire "GMA" team, saying, "We're the face of it, but it's all you. Everyone in this building is a team."

We're guessing the "Today" staff got bagels.

Katie Couric's debut as a fill-in host this week on "Good Morning America" has pissed off some of the GMA staff, who feel it's a slap in the face to vacationing co-host Robin Roberts.

GMA is breathing down the neck of "Today" and could overtake it in the ratings this week, breaking years of NBC morning show dominance. Katie's week-long stint -- along with the fact that millions of people have their TV sets tuned to ABC because of "Dancing with the Stars" -- could put it over the top.

Our GMA sources tell us it's disgusting that honchos chose the week that Robin was on vacation to put Katie on the show. They note Robin has been co-hosting GMA since 2005 and is a big reason the show is doing so well.

As one insider put it, "They should have put Katie on when George was on vacation."

And the contingent of GMA staffers who are pissed off is large -- because Robin is well-liked. And, sources tells us, although Robin is unhappy with the Katie move, she has not badmouthed anyone.



As for Katie, ABC sources say she had nothing to do with the decision to fill in and she was very conscious today that she was a "visitor on the set."

"Good Morning America" weatherman Sam Champion got REAL excited for the forecast today -- accidentally drawing a full-blown penis on live television this morning ... and it's hilarious.

In case you're wondering -- yes, we are 10-year-olds.

Kris Humphries is telling people he felt ambushed by "Good Morning America" this morning -- claiming he was put under the impression the interview was NOT going to focus on his split from Kim Kardashian ... but "GMA" is calling BS.

Sources close to Kris tell TMZ ... the NBA player claims he knew he would be asked about the divorce during the sit-down ... but he didn't expect to be grilled like a cheeseburger.

During the interview, Kris appears VERY uncomfortable as he's hit with question after question about Kim ... and things got downright awkward when interviewer Josh Elliot asked Kris' mother about the relationship.

We're told Kris is saying ... he only agreed to do the interview to promote his charity work and to support his mother's cooking segment on the show ... and he's livid about the way it turned out.

But a rep for ABC tells us no pre-show deal was made to limit the interview ... saying, "There were no ground rules."

George Stephanopoulos could lose his "Good Morning America" anchor chair within a year to the news reader who towers over him ... this according to network sources.

The buzz at ABC is that Josh Elliott -- a former ESPN sports anchor who has been the GMA news reader since March -- is on the fast track for the anchor seat.  Josh filled in on "World News Tonight" last week and network suits are in love with him.

The network will deny it's contemplating a change, but that's not what we're hearing.

The ratings at GMA have actually gone up a bit with George, so why would the network mess with the show?  Our sources say the suits think Josh and Robin Roberts have better chemistry and look better together.

Our sources say the buzz is that George could go back to his old job, anchoring "This Week," especially because execs privately now admit Christiane Amanpour is a disaster as an anchor.

Now why, you ask, do they look better than Robin and George?  Robin is 5'10" without heels.  Josh is around 6'3."  George is 5'6" ... and a half.  There's a lot of talk at ABC that it just looks awkward when the male anchor is slight and short. 

Not that there's anything wrong with it.


7:55 AM: An ABC News spokeswoman tells TMZ, "This is Total Nonsense."