There were some clues as to who's identity it was, they said so in the Talking Dead so it's legit, right? LOL
1) Well, when Negan says to cut the kids (Carl's) other eye out and feed it to his dad if anybody moves, he uses his right hand to point, as if Carl and Rick are sat on Negan's right hand side, also at about the same time he turns to the right, something you probably wouldn't do if stood on the left side of the group (Negan's right) as you'd be talking to the trees, not the group.
2) When Abraham sits upright, Negan laughs, as if to say 'respect', then after the first hit, Negan says something like 'yeah, taking it like a man', this could mean two things, Abraham indicated he was prepared for a beating (by sitting up) then when it actually happens, whoever it was sits up again, like he did, but this could also just as easily apply to a woman.
So who was on the left side of the group as Negan looked at them?, iirc it went Glenn, Rosetta (or was it the other one?), , Michone, Daryll, Abraham, Maggie, Rick etc.
Yup, I think it was Abraham, he was spared the arrow a few episodes ago while out looking for the bullet factory like what happened in the comics, so to avoid a Darryl type scenario with a recurring character, and to avoid the potential backlash from the fans (something they seem to care about), it seems easier to kill off Abraham at this point, and spare Glenn, both Glenn and Maggie can still run the Hilltop like she does in the comics, and it saves a lot of potential grief of having to write for yet another character already killed off in the comics, as opposed to completely alienating the most vocal of your fanbase.
However, the producers have also said that Andrea's storyline from the comics will be split across several characters, not just one, so who knows, could it be Michone? Whoever it is, even the cast don't know at this point....