What made Chandler unique was his ability to hassle quick guards who got into the paint. The Mavs defense against speedy guards was so horrendous that having Chandler was a must.
This year, that's not the case. West and Roddy both have proven more than capable defenders against speedy guards, and it has helped the defense tremendously. Don't get me wrong, I would still love Chandler to be in there, but losing him didn't hurt nearly as much as it would have, if West/Roddy weren't in the rotation. Also, it seems like Carlisle has changed up the defensive system a bit, to attack passing lanes for steals, and to swarm the paint quicker. I think the increased level of athleticism across the team has allowed these types of schemes to work better, as the perimeter defenders are quick enough to recover and still get back to their man or rotate to the perimeter (although the last couple games they haven't defended the perimeter that well)