And like half the side characters. But that part isn't a huge deal, it's relatively easy to find people who can do the voices well enough. This guy alone could probably do it and because he's a nobody they could pay him peanuts, although they would be better off casting someone else to do Rick.

The bigger problem is he's the co-creator and much of the humor comes directly from him. Dan Harmon makes the show compelling and gives it plot and character development, but the bizzareness of it that makes the show unique comes from Justin Roiland. There are people who can imitate this and you can continue getting an OK show that will coast on it's success for a few seasons, but it will always feel a bit too formulaic and cheap without Roiland AND Harmon.