Cain was able to get back up from underneath Brock because
i) Brock rushed his positional advancement too much, not sure why but had he been calmer he would of stood a better chance of keeping Cain down;
ii) Cain is a division 1 level wrestler who has trained in wrestling pretty much non-stop for years and trains at an incredible camp.
However, no one Brock has face, i repeat NO ONE, has been able to prevent being taken to the ground. That includes Cain, Couture and Carwin. Junior is going to hit the ground - i sincerely doubt he will be able to get back up from underneath Brock. And if he cant, he is going to get manhandled similarly to how Mir did.
I see no reason to believe that Junior has TDD superior to that of Cain or Couture, or a similar ability to get up from his back.
If he does manage to get up however, and that in my view is a BIG if, he will destroy Brock on the feet. Thats no secret. I just dont think he'll get the chance to.