Knowing how Rondo plays, this is exactly what will happen. He plays incredibly lazy regular season basketball (except when he is on national TV), which is why our regular season record has been pretty weak since he took over the reigns as the leader of the team. But in the post-season, he is arguably the second-best player in the world (only to Lebron, obviously).
This injury will likely cause hustle players, like Lee and Bradley, to play their hearts out and rack up regular season wins now that they don't have a point guard that doesn't care about the regular season. I think they could end up with the 3rd or 4th seed. But in the post-season, they don't have anyone that will step up the way Rondo does. I predict a 2nd round exit, but the rest of the regular season will probably cause some fans to feel like we're better without Rondo (which is incredibly idiotic).