Denver have a surplus of big men. I could see them making one of these guys available midway through next season at a cheap asking price.
I think they get a first rounder for Koufos or a pair of second rounders for Mozgov (or a comparable role player talent wise, probably a wing). Karl seems higher on Koufos so I expect it'll be Mozgov that they look try and move.
The main issue with Mozgov though is that he still needs quite a bit of development. More of a band aid than a genuine solution to the Celtics need.