Lesnar has a glass chin. If Jones keeps his distance and uses his length, he may KO Lesnar and at 40 years old Lesnar is slowing down. But, Lesnar is huge and may provide a challenge for Jones.
Finally, the UFC should never allow Lesnar to fight in their organization again. They let him fight vs Mark Hunt knowing he likely was on something. They even waived the testing rules in order for him to fight. I would suspect Lesnar is still on the roids. Not that Jones is a clean fighter given he has a tumultuous history and has been given chance after chance.
Finally, this would be a stupid fight in my opinion.