The only team I see the "downgrade" from DRob to Rasho affecting us is against LA. And that's not even a sure thing, since Rasho will hit that midrange shot with more consistency than DRob did the last couple of years.
Against all other teams, this is an upgrade, and probably a significant one. Unless Yao Ming turns into a dominating force, there really is no other team in the league where Rasho's man-on-man defense will be a big factor in the game. Sacramento won't be running plays for Brad Miller. Raef probably won't see the ball much with the other 4 options on the floor. Same story for Kandi. Tsakalidas?
Against teams who didn't often go to their center, Robinson's talents on defense were not as necessary. His help defense was still superb, but Rasho has proven he can block shots as well. On offense, DRob was a liability that Rasho will not be. We now have 4 legit offensive weapons on the floor to start every game, as opposed to the 3 we had last year.