Honestly, like Keldon, Zach isn't playing back as much as he's also struggling with the way Pop has run the rotation. I never loved him as the starting big next to Wemby -- and I think the Spurs probably misread the whole situation by assuming he would be. Zach is a single big who needs speed, movement and spacing around him very similar to Wemby. Maybe in a few years when both are seasoned, they could learn to co-exist the way that Duncan and Splitter did. Right now, it's not there, and both would be better off playing most of their minutes apart. I have little doubt that had the Spurs not won the lotto and got Miller, Collins would be looking pretty decent right now.
I said pretty early that Wemby probably operates best with rim-runner/shot-blocker type rather than a finesse center like Collins. I wish the Spurs took that prospect seriously rather than seeing that archetype as an afterthought and putting zeroes like Bassey, Bediako and this new two-way there instead. They seemed way too confident in their prediction and have ended up with a decent player who probably can never justify his contract with the makeup of the roster now.