Rondo is an outstanding point guard. His role is clearly defined here. In this system... right now... he's a good fit. The other 4 starters (Ray/Bradley, Pierce, KG and Bass) are all capable of hitting shots (Bradley is a recent revelation that hopefully isn't just streaking)... so Rondo can get away with being a poor shooter and focus primarily on being a setup man. He excels in the role. If his teammates keep shooting the way they are shooting, Rondo should have no trouble keeping his assist numbers up.
I think these stats might just be skewed by the fact that when Rondo missed games, we won 6 of 8. The thing is, even with how well Rondo plays as a "setup guy", he's still got the potential to be a "weakness" for the Celtics due to his lack of offensive ability (can't shoot, can't drive late, because he can't hit free throws)... so when he sits out, our star players have a tendency to put on their vintage capes and dominate. Pierce, in particular, seems to step his game up when Rondo is out. Pierce seems more than comfortable letting the ball flow through him (which is how he had to play up until 2009, really)... I'm not sure if that proves that they don't NEED Rondo out there... I suspect that even without Rondo our offense would thrive. Some have theorized that if you stuck a shooter in there like Jose Calderon, the offense might actually be better. It would require Pierce to control the ball more, but he'd have more space since Calderon was such a deadly shooter. WE'd take a hit on defense, though. Rondo has proven to be a very good defensive point guard when he's focused.
Edit: Just took a look at what Pierce did in the 8 game stretch with Rondo out and it seems to back that up:
1/20 (LOSS) 12 points, 6 assists, 3 rebounds, 2 steals, 2 blocks
1/22 (Win) 34 points, 10 assists, 8 rebounds, 3 steals
1/23 (win) 19 points, 7 assists, 5 rebounds
1/26 (win) 24 points, 10 assists, 6 rebounds, 1 block
1/27 (win) 28 points, 8 assists, 10 rebounds, 3 steals,2 blocks
1/29 (loss) 18 points, 5 assists, 6 rebounds
1/31 (win) 20 points, 6 assists, 4 rebounds, 3 steals
2/1 (win) 17 points, 8 assists, 6 rebounds, 1 steal, 1 block
Pierce averaged 21.5 points, 7.5 assists, 6 rebounds during that stretch. So basically what happens when Rondo sits is that Pierce gets asked to do more on offense (which he's capable of doing... dunno if he could do it long-term at this age, though) and we replace Rondo with someone who can shoot better (pretty much anyone... by default). That doesn't take away from what Rondo does, though. He's a very skilled setup man and in THIS system RIGHT NOW... he's clearly flourishing in his role.