The argument I'm about to make is pretty simple in essence: Luol Deng has been the best defensive player on the best defensive team in the league and should therefore, at the very least, receive some consideration for defensive player of the year.
First of all, it's pretty reasonable to say that the Bulls have been the best defensive team in the league this year. The Bulls have a 100.1 DRtg this year. For those of you who don't know, DRtg is an all-encompassing statistic which records how many points the opposing team scores for every 100 team possessions. It accounts for the whole shebang: field goal percentage, free throw attempts, turnovers, and rebounding. The Bulls are ranked 1st in the entire league by this measure (the Celtics are a close 2nd at 100.5 and the Magic are a distinct 3rd at 102.0).
Now with that in mind, the next topic of conversation is determining which player on the Bulls has made the largest contribution to the Bulls' defensive excellence. Ask a random Bulls fan who the Bulls' best defensive player is, and I'm guessing the popular answer is going to be Joakim Noah. Now, I love Joakim Noah as much as the next guy, but it's pretty hard to make much of an impact when you're not on the floor. Noah's missed quite a few games now with his hand injury, and the Bulls have actually played better defense as a team in his absence. You can chalk a lot of that up to the fact that the Bulls schedule was much more difficult in the early part of the season, but the point still stands that the Bulls haven't really missed Noah that much defensively.
As I said previously, it's pretty hard to make much of an impact when you're not on the floor. Scroll over to your favorite basketball stats-keeping website and you'll see that Luol Deng has led the Bulls in minutes played this year with 1570. The only other Bull that's even close is Derrick Rose with 1476. Ronnie Brewer, Taj Gibson, Joakim Noah, Omer Asik and Kurt Thomas have all played brilliant defense at times this season, but it's pretty clear that none of them have been as impactful as Deng, because none of them have been on the court nearly as much. As for Rose, the strides he's made defensively have been wonderful, but he still has a ways to go to be called an elite defender. Besides, it's very difficult to have a large defensive impact as a PG for the simple reason that the PG spends so much time out on the perimeter that he can't really do much in terms of protecting the paint, which is what good defense is all about.
So with all that said, I'm now going to talk about Deng's defense specifically. I'm not going to attempt to extensively use advanced statistics to support my point for two reasons. The first is that attempting to quantify a player's defensive contribution is something that is incredibly difficult to do; all existing defensive statistics are at least somewhat flawed. The other reason is that, from my experience, most of the people who buy into advanced statistical analysis are already large supporters of Luol Deng, while most Deng-bashers prefer more qualitative analysis.
Anyways, Deng's defense this year has been undoubtedly superb. He has tremendous length which makes him one of the best at contesting shots. His fundamentals are outstanding; he knows where he's supposed to be and what to do when he gets there. If anybody doubts that, there's a reason that Tom Thibodeau, who has a pronounced reputation as a defensive-minded coach, loves the guy and plays him more than anybody else. Deng routinely guards the other team's best perimeter player (except when he's playing the 4, but more about that later), and generally does a very good job (see today's game vs. Memphis). He's one of the best rebounding SFs in the league, which in and of itself is a very important part of defense, but additionally, his ability to rebound and guard PFs has given Tom Thibodeau a lot of lineup flexibility, which is what allowed the Bulls to largely avoid playing Brian Scalabrine while Carlos Boozer was out.
So there you have it folks. Luol Deng's a great defensive player who's probably been the biggest reason for the Bulls defensive excellence this year. He doesn't get nearly as much credit for his contributions on that end as he deserves, and hopefully this post will change that somewhat. I don't really expect him to win defensive player of the year any time soon as it's extremely hard for a perimeter player to do that, but I'd like to see him, at the very least, be in the discussion.