This fight does not interest me. Penn and Shields are far better grappler's than Diaz. Penn has far better Boxing and pretty much everyone GSP as fought had better wrestling. Diaz has nothing here, he is a weak wrestler and he isn't gonna get the sub from the bottom.
Penns BJJ from the bottom is far too defensive. Diaz has an aggressive guard, he's a lot bigger than BJ Penn, he has much better cardio than BJ Penn, believe it or not, he has better boxing than BJ Penn, he has a 3-4 inch reach advantage over BJ. There are lots of things which go in Diaz' favour here. Diaz is better from his back than Shields too.