Survive the first rd and he has a great chance to win. but really,he may only need to survive the first couple of minutes since Overeem hasn't fought in over a year. hell, he was gasping for air after the Werdum fight and he barely did anything.
And Bigfoot isn't as slow as people make out. sure he isn't fast but he has quite good handspeed and explosiveness for someone his size. Overeem isn't a whole lot faster either. if he is even faster at all.