First let me stress that I'd be thrilled with a first round of Moe Harkless and Fab Melo.
I don't think Melo would be a reach at 21-22, and I think he'll be a very decent all around center - big enough to body up and block shots, quick enough to rotate and tip rebounds to teammates, fast enough to run the floor, skilled enough to post up sometimes, and a decent enough stroke to stretch the floor.
I also think Harkless and Melo both have PF versatility in certain situations, Harkless defending undersized or thin stretch 4's, and Melo drafting big bodied 4s like Kevin Love.
That would be an excellent, excellent draft.
However, if the draft plays out in the worst case scenario, in which Harkless, Quincy Miller, Moultrie, and every other high ranking 3-5 go in the top 20, we may or may not be well served to trade down with Cleveland.
In this scenario, Fab Melo could be drafted 21st, and Cleveland could be looking to take a wing, if Cleveland drafts big in the top 10.
Cleveland could trade picks 24 and 33 or 34 (they own both) for pick 22 and 51.
This scenario would require Cleveland to be very high on a particular player, and worried that he would be taken at 22 or 23, just before they pick. Since they own 2 high second rounders, trading back to 51 might not seem like too high a price to move up to 22 and get their guy.
For the Celtics, Royce White (I don't trust him), Andrew Nicholson, and/or Festus Ezeli, along with several quality smaller guys would still be available at 24, and moving up high in the second would greatly improve the likelihood drafting an impact third pick.
Like I said, hopefully we end up with Harkless or (less fortunately) Q Miller at 21.
Just throwing out this trade down scenario because I have way too much time on my hands:
21 C/PF Melo
24 PF/C Nicholson or PF/SM White
34 SG John Jenkins or SF Darius Miller