Robin Roberts will not re-join the "Good Morning America" staff until May at the earliest, and more probably June, 2013 ... TMZ has learned.

Multiple sources connected to the show and the anchor tell TMZ ... Robin's medical issues make an earlier return way too risky, because her immune system is extremely weak.

We're told doctors are pleased with her progress and say she's right on schedule, but that schedule puts her on the bench for 5 or 6 months ... something she's not crazy about but realizes is necessary.

Sources at GMA tell us ... the honchos are hoping that Robin can return as early as February, but we know that's not in the cards.

And speaking of cards ... we're told they're coming in by the truckload -- cards, emails, etc, wishing her well. GMA honchos believe Robin's return will boost ratings for the show even higher, which can become problematic for the struggling "Today" show.