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While Junior dos Santos will surely get people excited after the sustained thrashing he took from the champion not a year ago, it's hard to peg him as the biggest threat to the Cain Velasquez era (this one, at least).
Other guys like Josh Barnett and Alistair Overeem are curious options, but are a few fights away at a minimum.
Enter Fabricio Werdum, the one style the world has yet to see matched with Velasquez and the one that may well give him the most trouble.
On his feet, Werdum is incredibly dangerous, a true practitioner of using eight limbs to maximize his output. And on the ground? Well, on the ground he's racked up six ADCC medals and submitted Fedor Emelianenko and Minotauro Nogueira within his past four MMA wins.
A guy with submission skill like that isn't going to be afraid of the inevitable ground fight with Velasquez, and his stand-up is a wash with the champion. At worst.
This is the guy to challenge the heavyweight champion in a way the world has yet to see.