I do think it depends on where he falls. Doc makes a fantastic point in that there are not a lot of guys who can create their own offense in the NBA. Look at the Celtics now - Pierce can. Garnett really can't, Ray can't anymore, Bass, Green, Pietrus, Bradley, etc. - none of them can create their own offense. With Rondo, it's arguable he can create his own offense, but it may just be that he can only create for others (if it's one or the other, I'd say creating for others is somewhat more valuable).
Thing is, if you're picking in the late teens and you can get a guy who can not just score, but create his own offense, that's probably more valuable than anything else you can get.
The temptation, if you got Rivers, would be to trade him to Indy, say in a sign and trade for Hibbert and the rights to Indy's first rounder and/or a side deal bringing Collison to Boston. Collison-Ray/Pietrus-Pierce-KG-Hibbert to start, Bradley-Rivers-Green-Bass-Stiemsma off the bench.
But like I said, if the choice is a guy who creates only his own offense or a guy who creates for others, I prefer the guy who creates for others. KG, Hibbert, and Ray/Pietrus are a lot less effective if they don't have a true playmaker setting them up. And Pierce can't be expected to continue doing that - his offense is going to become more limited in the next two years.
So if you're gonna take Austin, I'd rather keep Rondo. Green becomes a viable starting small forward option next to those two in the next couple of years, since when Austin can move into the starting lineup, he and Rondo will handle the ball all the time. A guy like Green who gives you good perimeter defense without needing the ball in his hands becomes more effective. In the short term, I'd think Doc would want to keep Ray here to mentor Austin, and to have that veteran crew in place to avoid the locker room getting poisoned by notions of favoritism toward his son (the possibility of locker room conflict stemming from that weighs in favor of trading Rondo, but could be mitigated if Ray and KG are still here). You run out a starting five of Rondo, Ray, Pierce, KG and another big - you'll have to hope you can get somebody with the MLE, and bring Bradley, Austin, Green, maybe Bass and Stiemsma off the bench. You still have Jajuan, Moore and whoever else was drafted (a first and a second rounder) on the deep bench.
I think it can work. If he is the best player available. And given that he has the talent to create his own shot - even if that talent still needs to be developed along with his defense (not that I think his career will reach their levels, but Jordan, Kobe and Lebron were all poor defenders coming into the league and became elite on that end) - I can't imagine he wouldn't be the best player available if he slips to the 17-20 range.