No disrespect to Johnson. If you are unaware, look up some footage of the headbutt. There is one thing that took Gustafsson down, and it's the headbutt. Plain and simple.
Obviously Gustafsson didn't recall a headbutt in his post fight comments. I'm sure most people disregarded a headbutt, including myself. I feel after seeing this replay, the breakdown of the fight changes drastically. How can you suggest it was a big shot that took down Gustafsson? For goodness sakes you can see his hair whipping back from the impact!
After seeing the replay of it, I feel very comfortable saying a rematch is the best answer. There wasn't anything intentional, however when a fighter drops as a result of a headbutt and then in the same sequence is finished, what is a fair resolution?
I think it's going to take some momentum before this even becomes a thought for the UFC. Nobody talked about it after the fight, but now that the footage is being seen, people need to voice their opinion. Let's just say that the UFC actually dropped the ball on this one because I certainly don't recall anyone showing the replay or the mentioning of a headbutt. That would be the commentary team I'm referring to. If this was for a title and the headbutt got missed, I think this would be a far great controversy. It's just getting starting imo and it's because of the TV coverage or lack thereof.
If you don't think this is enough to warrant a rematch, did you actually watch the replay going around on the internet?
Update 2: See Juupa's supplied GIF in comments for a more accurate account of events. At this point, it's safe to say that a majority of the "felling" impact was from the overhand strike and not directly from the heads colliding. Should there be a rematch? It's not as compelling an argument at this point. Lesson here? Make sure to see clearly all possible replay angles prior to having a final conclusion on what happened.
Update 1: To stop the GIF caused misguided opinions, please view this video of the sequence, and then let's discuss.