Where are you getting those stats from? NBA.com has Exum #1 for net rating. Technically Naz is #1 but he didn't qualify.
Exum runs the offense fine. He did a decent job of getting the offense as a rookie. He got the team into it's sets and moved the ball to where he should.
I think Exum gets 4 years $40 mil and it's a great deal for the Jazz.