Jefferson is a player that's given the Bobcats trouble for a while now due to his impressive array of skills in the post on offense. However, from the other side, Jefferson has given his own teams trouble due to his defense.
His offense is a remnant of an era past considering the dearth of true post threats today. Nearly 46 percent of his offense comes on post-up plays, on which he scores 0.89 points per possession. That's good enough to rank 44th overall in the league on such plays, but it's one of his least efficient plays, according to MySynergySports.com. Jefferson is also solid on pick and rolls and on cuts, though he didn't see as many of those plays as spot-ups. Overall, Jefferson's an efficient scorer with 0.97 points per possession (ranked 102 out of the entire NBA).
As mentioned above, it's his defense that causes more issues. Jefferson just doesn't defend well on the pick and roll; he can't hedge to contain point guards. He's slow. It's a bad combination.
To make matters worse, statistics measuring team performance specifically with Jefferson on the court versus when he is off it indicate that he has a negative impact overall. The offense gets marginally better with him there, which is to be expected. Team effective field goal percentage (eFG) improves from 47.9 to 49.9 percent and turnover percent decreased with Jefferson on the floor for the Jazz. Despite this, overall offensive rating (points scored per 100 possessions) stayed about the same.
On defense, it's scary. Opponents shot a 51.7 percent eFG with him on the court, compared to 47.1 percent when he's off it. They don't turn the ball over as much when Jefferson's there. Points per 100 possessions for opponents jumped from 100.7 to 110.5 when Jefferson took the floor for the Jazz last year.
I don't think all of this can be placed on Jefferson - Utah's defense was hardly the most disciplined machine - but it should be noted that teams with Al Jefferson historically struggle on offense.
However, no team has ever been able to play decent defense while starting Al Jefferson, and that hasn't changed now that Jefferson is in Utah. Starting in the 06-07 season, when Jefferson first became a starter for the Celtics, here are the number of points per 100 possessions Jefferson's teams have given up when he was on/off the floor:
06-07 Celtics: 108.5 with Jefferson/102.8 without Jefferson
07-08 Timberwolves: 116.0 with Jefferson/103.9 without Jefferson
08-09 Timberwolves: 112.3 with Jefferson/113.2 without Jefferson
09-10 Timberwolves: 113.1 with Jefferson/111.2 without Jefferson
10-11 Jazz: 112.0 with Jefferson/101.3 without Jefferson
As you can see, the only teams that didn't play significantly worse defense with Jefferson on the floor were the 08-09 and 09-10 Timberwolves, and that was only because they were so bad defensively Jefferson wasn't able to do much damage. +/- is a very dangerous stat, but Al Jefferson has started for six seasons for three different teams, and all of them have played horrible defense when he is on the floor.
- ProBasketballTalk (2011)
The thing Jefferson has going for him is a big one, though. He's a talented threat capable of shouldering an offense in the post, drawing double-teams and not turning the ball over much. Whether it outweighs his defensive liabilities, I just don't know. Charlotte's already a godawful defensive team so it's not like Jefferson would make them worse, but if the team wants to climb out of the cellar of disastrous defense (and they do), I don't think it's going to be with Al Jefferson in the paint.
I'm also not really sure what the Bobcats are going for here. If they want to bring him in to put Bismack Biyombo into the backup center slot, the team has an offense-heavy frontcourt with Cody Zeller and one that will undoubtedly struggle to defend anything at the rim. Or they could push Zeller to the backup slot and Biyombo slides to power forward, which could help with Jefferson's lack of defensive talent. But how long until you wonder if you're holding back Zeller's development? I don't really get the possible fit.
If nothing else, I think this is the Bobcats trying to say they're serious about big name free agents.