Pau doesn't make sense for this team at all. He doesn't get Boston anywhere near a title contending team next year and he is too old to be a component/asset/whatever going forward. I just don't see wrecking the draft position this year for a guy like Pau even if you do get out from under Wallace (who the Lakers would never take anyway so the it isn't worth discussing).
This trade would never be about Pau making us a title contender. It is about swapping parts so that both rosters make a little more sense. I actually don't think the Celtics are going to tank but think about playing well in a different perspective.
If they can start the season with Wallace (I have a different Pau trade where we send Hump., Bass and Melo) and Pau starting and actually play well (as individuals and as a team). What would happen is that you increase their trade value, in essense increase the equity we have in our "assest".
Then you trade Wallace for some value (and maybe Pau too but he would be harder) and then start to tank. You might end up getting the best of both worlds. You get some value for Wallace and you get a high draft pick.
I actually like the Pau trade and have suggested it in other threads.