If you think Jalen Smith can play at the 4, you kind of have to give the same benefit of the doubt to Nnaji. The idea is that he'd give the Jazz a big, mobile defender who can also space the floor and play backup 5. So the Jazz would bring him in to replace Derrick Favors' role. That said, it's in the same vein as Jarrell Brantley and Juwan Morgan. So the question is how much better Nnaji is going to be.
FWIW, I still think Reggie Perry is more offensively skilled than Nnaji, but the risk with Perry is that he plays defense as an afterthought. Nnaji is a disciplined defender. He'll dig in and do the dirty work that Favors used to do. He's got good size at 6'10" w/ a 7'2" wingspan.