Why in the world would the Celtics even consider doing this?
Trading Rondo for Nash and a pick, no clue.
Using the trade exception to take on Nash in exchange for a future 1st-rounder? If they've over the cap but not over the luxury tax, and can't find a better way to use the exception, there are worse things they can do. Sure, Nash doesn't have all that much left in him, but I imagine he'd be better than Pressey as a backup point guard. And a future asset, even if it's a 2020 1st rounder, is better than no asset at all (which will be the case when the trade exception expires July 14th).
I'd prefer the exception get used in a different way, but said ways (like trading for Asik) may not be possible.