It's MMA so anything is possible (see Serra, Matt), but I just can't see a way for Diego to logically win.
Standing: Diego is a brawler with very little in the way of technical striking. BJ is one of the most technical boxers in the division. If BJ has his jab going like he did in the Sherk fight it's going to be a long night for Diego. Also don't go looking for a lucky shot, BJ's chin is epic.
Wrestling: Diego's GnP is his best weapon unfortunately for him he has poor technique on his takedowns. Even worse news is BJ has arguably the best take down defense in MMA. BJ doesn't have great take downs, but he does a good job of using his striking and timing to setup the double leg.
Grappling: BJ has the best ground game in the division by far. He's never submitted anyone from the bottom, but I don't think that's from lack of ability. He rarely goes for subs from the bottom, he just uses his excellent guard to get back to his feet. Only 2 guys have really been able to keep BJ on the ground and they were arguably the 2 best top fighters in MMA history (Hughes and GSP).
So yes, this is MMA and anything can happen, but that doesn't change the fact that Diego is rightfully a big underdog.
Toxic approves this message
Sums it up nicely BJ is better at every aspect and being the bigger and stronger LW isn't gonna matter when your fighting a former WW champ, BJ handed beatdowns to bigger stronger fighters than Deigeo.