It really is a simple matter of minutes and roles.
First, with the small ball line-ups and the minutes played by Duncan and Horry, there has been only 10 to 12 minutes of time for a third big man. Pop even said last week that there were only minutes for 3 bigs and Nazr is the odd man out. In order to play Nazr, from whom do you want to take minutes? Surely, not TD and Rob. Is it simply a matter of wanting to give Rasho's minutes to Nazr? This is the main reason why that is not happening. The minutes Rasho is playing are primarily to give Tim a rest. If you look at the minutes played together recently- in the last 3 games- Rasho and Tm have been on the court together all of about 4 minutes grand total. Primarily, Rasho has played with Horry or as the lone big with 4 perimeter guys. You can't go small ball with Nazr as the lone big. Pop tried several times this year and it just does not work. The problem for Nazr is that he NEEDS to play alongside Tim and those minutes simply do not exist for the third big man- they are going to Horry or a guard, like Brent or Michael. The Spurs have seen that defensively their best option is to have either Tim or Rasho on the court at all times. And that is what they have been doing. Haven't you noticed that it is almost always Tim and Rasho subbing for each other? Nazr just does not fit into the roles they need right now. As long as the Spurs keep winning with Horry starting, and the Spurs choose to run some time with the small line-up, I don't really see things changing much for Nazr