I'm optimistic about Jennings improving his efficiency. He has shown good development with his jump shot since joining the NBA. And he's a gym rat.
Jennings is a good scoring point guard but he's also a talented passer and playmaker. I wouldn't classify him solely as a scoring PG (like Mo Williams). I also thought he did a nice job directing Milwaukee's offense and making solid decisions ... helping facilitate ball movement. Not just passes that resulted in an assist.
How does he compare to Rose?
Rose and his scoring efficiency?
Rose doesn't get to the line and doesn't take many three pointers. Those two traits will always limit his efficiency. I'd expect him to get better at getting to the line, maybe by 20% or so, over the next couple of years. The three point shot is a major unknown but assuming a low level of use (2 3FGA per game) of efficiency (33%) ... that would likely result in a TS% of about 55%.
It's rare to see guys improve their vision/creativity so it's unlikely Rose shows much improvement there. He should (will?) get better as a decision maker and as a floor general. He is still weak in that regard and benefited from Kirk Hinrich's presence as a starting two guard giving Rose a secondary ball-handler to help facilitate the offense.
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The main advantage Derrick Rose has is his overwhelming athleticism. His quickness is incredible, he has good size and excellent strength. It is very difficult for any opponent to gain a physical advantage against him (helps him defensively) and Rose is able to create a physical advantage against most of his opponents. That means he'll be very consistent and difficult for opposing point guards to shut down (and prolific - scorer).
In other words, he'll create a lot of mismatches and very few PGs can create a matchup advantage against him.
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My expectations for Derrick Rose are ... Rose should be a very good scorer with solid efficiency + one of the best defensive players at his position + a good rebounder + a good average passer.
Rose's main positive attribute as a passer is the same as his scoring - his athleticism - that allows him to beat his defender with regularity, get dribble penetration, force the defense to collapse and then either score or make the pass.
So while his passing ability otherwise leaves a lot to be desired ... his ability, and frequency of, to break down a defense makes him slightly above average as a passer in my book. And in time, with the improved decision making and floor leadership he should becoming a good passer (but likely never a high assist man due to his lack of vision).