It's not about physical resemblance so much as shared anatomy. For example, while a human looks nothing like a kiwi and evolving half a world away, they're both bilaterally symmetrical, have four limbs (even if the kiwi's fore limbs aren't immediately obvious), two eyes, a cranium and a vertebra. All these features are there because of shared ancestry that is much closer in evolutionary terms than say, a starfish. Even a Shark has most of those features in common, again, because of shared ancestry.
That the Na'vi appear to be the only large animal with two eyes, four limbs and only one tendril link thing means that they're either the result of adaptive engineering, or they're from a very obscure branch of the evolutionary tree. Indeed, the fact that we do see the Pandoran equivalent to primates and they have six limbs and four eyes is very suspicious indeed. In mean what are the odds that two totally separate evolutionary lines, divided by something in the order of a billion years or more and yet ended up co-existing while sharing a very similar yet distinctly separate anatomies?
Imagine if primates on Earth were all hexapods and then try arguing that we're related. Something fishy is going on.
Indeed, hasn't Cameron said that he intends to visit other planets in that system? Perhaps we'll find relatives of the Na'vi on those planets too.