Theres nothing in the CBA that says Bird rights are lost for a player traded for using a DPE. This article from MassLive says they would get Bird rights for a player they traded for:
link
I think the confusion stems from the reports that the C's likely wouldn't trade for Noel using the DPE because he would lose his Bird rights, but that's because he signed a one year deal and would have to approve a trade. Any player in that situation would lose Bird rights in any trade, it has nothing to do with the DPE. Williams is on the last year of a three year deal, his Bird rights should transfer just like in any other trade.
Manz, apologies, you are correct on a couple of points:
1. Lou Williams would retain his
Bird rights. Not sure how much we would want to go over the cap on a 32yr old Lou Williams next season with Smart contract coming up, but its worth something.
2. Our 2nd unit needs some shooting that Lou could provide. Still, don't love adding a 6'1" defensive liability to our team when MaMo (as a 6th man) could be an internal solution to our bench scoring issues.
Also, like Tyreke Evans more, but Memphis isn't giving him up anytime soon and Clippers are in free fall. Lou's trade cost would come cheaper, his availability would be sooner and he would have Bird rights. What conveys if that Clippers 1st rounder is in the top 14 picks in '19 or '20?
At the end of the day
Danny & Co can stand pat with the DPE and not be in a rush with the team winning and exceeding all expectations so far. Hence he'll probably wait, post-trade date, to see the buyout candidates and address the Celtics needs then.