Robin Roberts -- Sidelined from Interview with Chad Johnson's Estranged Wife Evelyn Lozada

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.