Morgan is the guy from the premiere played by Lennie James. I can't recall if he whispered anything to Rick, but I was talking about what the CDC scientist whispered to Rick. His name was Jenner.
Morgan came into the picture because Rick was leaving a message for him (which he may or may not ever hear). Rick was almost going to tell Morgan what Jenner whispered, and then decided not to. That whole scene struck me as significant, and provides a clue about what Jenner whispered: something that Rick might think Morgan should know, and then change his mind about. Why would he do that, unless it's something startling and important, but also unbelievable?
And they're obviously not hewing to the comics, since Shane is still alive. They might take some stuff from it, but and leave other stuff out. So the comics are irrelevant to making predictions about what is or isn't going to happen in the series.
And my theory is different than what you put in spoiler code anyway, so I don't see how it's even relevant to this discussion.