I think it's ridiculous that Lesnar is in this fight, but I care more about rankings than PPV's so anyway...
Junior should KO Lesnar very quickly. He doesn't need to feel Lesnar out, he needs to just rush immediately with his hands and Brock will probably hit the floor before he gets hit cleanly.
Brock's fear will stop him from trying a takedown or any form of counter during Junior's attack, and if he runs backwards JDS will be swarming and Brock will topple over.
If it get's past that, I think JDS will probably end Brock's takedown attempt with punches, he is very fast and finds gaps well. As for his TDD, we haven't seen much but he is springy, and I think his whole style is subtley formed around keeping the fight standing. Could he submit Lesnar or defend repeated TD attempts? None of us know, I don't think he'll have to though.
I used to have hope for Lesnar but Cigano is more skilled, more experienced, and so far has forced his game upon a range of styles. Dos Santos is the better fighter.