I don't think there is any chance that Rondo is demoted to the bench when he returns to action. But Nate's play has brought relief to Celtics nation in that we have another PG who can fill in nicely when Rondo is on the bench (or injured). Perhaps this will result in a minor decrease in Rondo's minutes when he returns, though even that is up in the air.
Nate's role on this team has been important since the day he got here. He is not just instant offense like Eddie House, when he goes off he can be the #1 scorer on your unit. The key with Nate has always been aggressiveness. Obviously Nate & Rondo do different things better, but it makes no sense to take an all star PG, and arguably a top 3 PG, and send him to the bench.