Facts need to be faced. if Nava was any better defensively in right field, or hitting lefthanded pitching, it might be a different story, but he's only a little more versatile than Carp, still has to be platooned with against lefthanders just like Carp does, and the question next year is about left field, which Carp and Nava play about equally well. I wouldn't want either one playing right field for an extended period, which is really the only thing Nava does that Carp doesn't. Besides, Carp has a power tool that Nava can't touch. And sure, Carp has only really come into his own this year after several years of mediocricy, but that is, of course, also true of Nava.
I'd predict that if Carp was given the green light and Nava shipped out, he would be able to take over the staring left fielder's job and perform equally as well as nava, with a plus for every minus -- I don't think he'd be able to get on base nearly as consistently as Nava, which is something Nava has always been able to do and is the reason he'll probably land on his feet in another organization, but I do think Carp is the only one of the two that could hit 20 HR's so there's a tradeoff involved.