Yeah, the problem generally with the Celtics trying to sign anybody off the buyout market is that guys who get bought out want to play significant minutes. They want increased exposure on playoff teams so they can have a better chance at getting paid this summer.
The Celtics already have a lot of guys on their roster who feel like they ought to get more minutes. Players who get bought out certainly know that, so they're probably not going to be too keen on signing with the Celts.
The exception might be a big man, given the problems that Aron Baynes has had staying on the court. But even when Baynes is out, the Celts often play small and they already have Theis, Williams, and Yabu that Brad might trust more.
I tend to think that Dewayne Dedmon or Kanter would be an upgrade on any of the guy currently in the depth chart behind Baynes and Horford, but the same might not be true of, say, Marcin Gortat or Robin Lopez.