My guess is that Diaz figures Lacy is shot and that he watched the Calzaghe fight. He'll come out throwing a lot of punches, expecting Lacy to cover up and let him. Of course, Diaz is no Calzaghe, but Lacy is more shot than he was then. It would make me pretty sick to see it happen, but if he can keep the pace it may work for Diaz, as Lacy can't catch him with a hook with his gloves to the side of his head.
It is, on paper, a smart choice for Diaz. Lacy is shot and, unlike Vargas, he couldn't fight much when he wasn't shot. My expectation, and my hope, is that Lacy catches him at some point and wins. I think it would be a mistake for Diaz to let it be a slow fight but I'd bet there isn't much chance of him approaching it that way.