Alright, everybody here knows the Cavs are a pretty piss poor defensive team. But the why is somewhat questionable. Is it the backcourt? Frontcourt? Coaching? Just youth and inexperience? I'll be dissecting some numbers from Hoopdata.com to try and answer these questions.
The Cavs currently have the 4th highest defensive efficiency. This is not good. They are giving up 106.9 points per 100 possessions only behind the Kings, Bobcats, and Hornets. They have the worst opponent TS% and eFG%, second worst block rate, and fourth worst defensive rebounding rate and opponent free throw rate. The only thing stopping me from calling them the worst defensive team in the NBA is their ability to create turnovers, currently third best.
Distance: Makes (rank), Attempts (rank), % (rank) [rankings from highest to lowest, i.e. worst to best, so 1st is bad]
At rim: 16.4 (15th), 24.5 (23rd), 66.7% (3rd)
3-9 ft: 3.7 (10th), 8.8 (20th), 42.5% (4th)
10-15 ft: 2.7 (11th), 5.9 (20th), 45.5% (3rd)
16-23 ft: 7.1 (17th), 17.3 (24th), 41.1 (1st)
3 pointers: 7.4 (13th), 19.7 (17th), 37.6% (4th)
At first glance, wow. That's ugly. The Cavs are literally in the bottom four defenses of every spot on the court in terms of percentages. The overall distribution doesn't seem bad. The one thing I would say would be to try and force more attempts in the 10-15 ft and 16-23 ft areas. Having a low number of attempts in these areas isn't necessarily a good thing, as you want teams shooting more from the least efficient spots on the floor. Indiana and Chicago force the 1st and 2nd most 16-23 ft shots, and the 2nd and 3rd most 10-15 ft shots, respectively, and are two of the top defenses in the league.
Combining the numbers together, the Cavs have an expected opponents FG% (XOeFG%) of 48.9%, which actually isn't too bad (12th best). This does not match up with their league worst OeFG% of 52.4%, giving them a league worst 0.93 Defensive Ratio (XOeFG%/OeFG%). This tells me that its not coaching or the system. The Cavs seem to doing an okay job of getting opponents to shoot where they want them to, but they just seem to be making the shots. Bad luck? Maybe. Bad defensive players? More likely.
The numbers at all distances are pretty equally disastrous, so I don't think you can pin this on one specific player/position. Simply put: Everybody/everywhere needs to defend/be defended better. Would a shot blocker help? At the rim, probably. From 16 ft to 3 pointers, probably not nearly enough to significantly change these percentages. The Cavs are very young, and there is an extremely high correlation between the youngest teams and the worst OeFG% (four youngest teams are in bottom five), so I expect a decent amount of growth and improvement in this area as Kyrie/Dion/TT/Zeller get more accustomed to the NBA.
The other thing I wanted to talk about was the defensive rebounding. It surprised me the Cavs were 4th worst. Obviously losing Andy hurt. I wasn't expecting them to be good, but I figured they were about average. Tristan is good, Speights is good, Andy was amazing, Dion is bad, Zeller is bad, everybody else was about average so I figured it probably evened out. So I'll go through each (main) player in terms of rebounding.
Player: DREB% (rank among position) vs personal expectations
Kyrie: 10.8 (14th) a little better
Livingston: 10.4 (18th) same
Dion: 8.5 (45th) same
Ellington: 9.8 (33rd) same
Gee: 11.1 (53rd) much worse
Miles: 13.4 (7th) better
Walton: 15.5 (27th) better
TT: 21.6 (20th) slightly worse
Speights: 23.4 (11th) slightly better
Andy: 30.4 (2nd) same
Zeller: 16.5 (47th) same
The only player who really broke expectations was Gee. SF is someone who has to rebound and Gee simply does not. Now this could be because he guards PGs and primary ball handlers a lot taking him away from the basket and he is slightly undersized, but still. A lot of people think we need a 3 and D SF to round out the team. Defensive rebounding might be something we seriously need to consider for the position as well. A team with Zeller, Dion (or Ellington), and Gee on the court at the same time could be ugly in the future when it comes to rebounding (which is why I'm leaning more and more towards wanting Zeller gone and keeping Speights and Andy).
Quick draft note: These rebounding numbers make me want Nerlens (21.7 DREB%) and Porter (18.9) more, and Shabazz (8.6!?!) a lot less come draft time.