I think people backing Velasquez are giving too much weight to Lesnar's reaction to Carwin's punches. Carwin has so much more power than Velasquez that it's ridiculous. Sure, Velasquez dropped Nogueira, but we can all admit that Nogueira is a shell of what he used to be. After the Rothwell fight, everyone laughed at Velasquez's pillow hands. Carwin, on the other hand, has a long list of flash knockouts and badly hurt Mir with a few choppy punches from the clinch. Lesnar retreating from that man really has no bearing on this fight.
For the life of me, I do not envision Velasquez being able to hold Lesnar down for long, and he sure won't pound him out. Carwin had him against the catch, but Lesnar was still able to get up before the end of the first round. Velasquez did take Kongo down repeatedly and was able to control him fairly easily. However, Kongo's ground game has to be the worst in the division, yet Velasquez wasn't able to finish him. Velasquez got a very quick stoppage against Rothwell, but he'll have to inflict far more damage against Lesnar to end this fight. I just don't see it happening.
It wasn't just that Lesnar turned and ran from Carwin, it was his horrid technique. While Cain may lack the power to straight drop Lesnar like Carwin, he should be able to put hands on Lesnar in volume, or enough to get close enough to shoot on Lesnar.
Cain has much better ground and pound than Carwin. Again, he lacks the power, but Cain moves really well on the ground, strikes while moving, and has much better control. Cain should be able to control Brock pretty well if Cain is on top.
If Cain can stop Brock's double leg, which I think he can, Brock won't have any tools to threaten Cain outside of a wild right hand landing.