If that fight had gone on AA was going to knock him out. He was landing clean shots, including a knockdown not counted shortly before the DQ.

I think although it was an illegal punch, I would have like to see the fight continue. We see low blows all the time, but that doesn't end up in a DQ. Even if it's a bad one, we might see the fight stopped for respite (doesn't the victim have up to 5 minutes to recover? from a low blow?)

The call was made harder by the fact Dirrell rolled around and couldn't continue. It was a solid punch, but seemed more of an arm swing rather than something with snap.

Found this video of the fight (they have full length and highlights)

Abraham vs Dirrell Highlights - Boxing Highlights

I've only just managed to watch the fight, hence my late opinions on it