Actually koast, Pop says that Rasho is a very good defender (one of the best positional defenders on the Spurs). He knows his assignments within the team defense, rotates well, and does a good job of challenging shots. His main problem is offense and lack of overall aggressiveness.
Nazr isn't as good on defense fundamentally. However, he makes up for it with hustle. He also is much more aggressive on the offensive glass. His hustle and offensive boarding is what earns Nazr minutes. Defensive slips will earn him splinters.