I love Nance but he is the worst defensive player on the team by far according to cleaningtheglass.com. The Cavs are giving up 19.5 more points per 100 possessions when he is on the court than when he is off. The next worst is Clarkson at 11.1. Since they're usually both on the floor at the same time they are probably dragging each other's numbers down a bit.
On the offensive side the Cavs are 10.0 points worse with Nance on the floor. Adding in the defense they're 29.5 points better over 100 possessions when Nance is not in the game. The next worst are Clarkson and Porter at 12.9 and 12.1. If you go strictly by the on/off numbers Nance is by far the worst player in the nine-man rotation.
All of the Cavs' starters are in the positive, meaning the Cavs outscore their opponents when they are on the floor. All four reserves are in the negative. Obviously the Cavs' weakness is their bench.
That could change once Henson and Windler can play. Henson would be a big improvement over Nance, defensively at least. I have no idea whether Windler would be an improvement over Porter.
Nance looks like he could be a good defender with his length, agility, and jumping ability but for some reason it's not happening. I've noticed a lot of layups being made on him, possibly because he tries to avoid committing fouls. He frequently ends up getting called for a foul, anyway. When he's on the floor opponents make a lot more free throws. Nance is in the worst 4% in the league in that category, but that could be misleading because he comes in at the end of the quarter when opponents may already be in the bonus or almost there.
The Cavs give up a lot more offensive rebounds when he's in the game, leading to second chance points. The only Cavs player worse in preventing offensive rebounds is Porter. When Nance is in the game with two guards, Porter and Cedi they are at a big rebounding disadvantage which leads to extra shots for the opponent.
Last year Nance was a solid defender - opponents averaged 1.4 fewer points per 100 when he was on the floor. He was in the middle of the pack, so I don't know what's going on this year, but poor defense and rebounding by the rest of the reserves could be a contributing factor.