I didn't immediately react to it, but during the intro the words "souls" and "marrow" were used and there was something about Shadow Man's voice that sounded so awful familiar. By the end of the intro I went "Manny Calavera? Gotta be!".

But it wasn't. They are two different voice actors. I don't know why that disappoints me so much, but it feels like how when I recently discovered that James McCaffrey who voiced Max Payne did NOT voice The Jackal in Far Cry 2 (but somehow he did Edward Carnby in Alone in the Dark!?)

Did you also pick up the similarities between Shadow Man and Grim Fandango?

No, not really. :/ However, there are slight resemblance now that you're mentioning it, but I never thought about Grim Fandango when I first heard Shadow Man speak.