I think calling him a top-10 Center is stretching it a bit. I will say that Joel Przybilla is hands down the best (soon to be) backup center in the league.
Let's See: Howard Yao Oden Bynum Shaq Ilgauskus Okur Bogut Kaman Nene ... I'm having trouble thinking of guys I consider to be truly better than Joel (out of the true centers, not counting guys who are PF/Cs)
I think he should be in that list. To non blazer fans it would be a stretch because he'll never put up as many points as most of those guys do.
IDK if Nene goes on that list yet. What about Camby and Biedrins. I think the two of them are above Pryzbilla. Al Jefferson starts at C as well. As does Al Horford. Dalembert is another C. Okafor. To make an accurate list, you have to include guys who play the majority of minutes at C even if they're shorter or might be a "PF/C". However, counting all those guys, Joel is still in the top few spots of the second half (ie 16-18th best C).
I am not sure Przybilla would be untradable; there are probably only a handful of guys in the league I would give that title. That said, I think it likely that Joel would be undervalued in most any trade and that it would be extremely difficult to get back what we would be giving up in him!
Joel gives Oden the time to become a great center! Messing with that is insane. Greg is the hub of our future team. If Greg isn't amazing, guys like Lebron don't think about Portland to go and win rings with. Greg must be the best center in the league in the next two years. Joel gives him that safety cushion to grow.