Tough one to call. I can potentially see Fullmer spoiling Canelo with his awkward and unpredictable attack and then hustling his way to victory, but I can just as well envision Canelo discouraging Fullmer's attack with inside powerpunches the way that Lausse and Tiger did.
I think this is a fascinating fight, personally. I say that because it's a fight between two fighters who I'd put on similar levels, and their styles would really be bad for one another. I can see Fullmer's unorthodoxy and frantic pace pushes Canelo's gas tank to the limits and pulling out a clear, messy decision over fifteen, where Canelo's counters and defence see him dominate early before tiring due to Fullmer's pace. Once he's tired, I'm fairly certain Fullmer would dominate most rounds. Fighting for 30 seconds around vs Fullmer is a very bad idea. Canelo could also beat Fullmer though, by putting him on the back-foot, and out muscling him like Tiger, Fernandez, Turner and Lausse did. If he did this, I'd give him a better chance of controlling the pace to a degree where he can win. I think what's more likely is that Canelo pushes Fullmer back but fails to control the pace. Canelo tires around the mid way mark and Fullmer starts to do better. He lands more and more and the rounds getting clearer and clearer. The outcome is a controversial decision - which obviously goes to Canelo - and it is debated forever.
I'd be curious to see how Canelo reacts to Fullmer's style, which was very rough and unlike anything Canelo has ever seen. When Fullmer uses that cross arm thing, he is coming in with his left elbow and the top of his head. I figure that Canelo would get cut at least once trying to learn where to put his head.