What I don't get is why people seem to think Jonas would walk into the NBA and have an immediate defensive impact? I'd like to believe that this organization and those in the media who referred to Jonas being a quality defensive player were talking down the road (if not, I question the wisdom of their statements).
Very few rookies are anything but defensive liabilities. A combination of adjusting to the new speed and style of game, lack of experience and lack of NBA strength almost always means rookies are going to take more away on the defensive end than they add. Even some of the greatest defenders and superstars in the league right now, LBJ and Dwight Howard, hurt their teams in their rookie year.
We are seeing this with Jonas right now. He is a net negative on D (+/-), opponents are playing at a near all-star level against him (PER of approx 20), and his defense rating is sitting below the team average (107 vs 106) and below the league average (104).
This isn't Jonas bashing... this is, as Matt52 said, rookie life. We will likely see an improvement by years end (although sometimes those rookie walls hit players pretty hard). Most importantly though what we want to see is growth and a marked improvement next year, and then ideally a significant impact in his 3rd year.
This year though is all about experience. Its on the job training.
What I would like to see is Jonas' minutes maximized (although capped at no higher than 30). Allow him to use all his fouls in a game if needs be. But this team has 2 philosophies that, as it was in 2006/7, are not running parallel. Win games now and "develop" what is hopefully a future core peice of this franchise. While its not impossible to do, its highly unlikely both will take place simultaniously.