*Cough Horford Cough*
The only issue with Horford is that the Celts are already hapless on the boards. Adding Horford would require finding a very good defensive rebounder to put next to him.
Yes, Sullinger is a good defensive rebounder but pairing him and Horford would only exacerbate the lack of size and length in the interior.
Outside of Boogie, I don't think there's anything out there this summer that fills all of our needs at the 5 spot - two-way player, can stretch the floor, protect the rim, rebound, etc. Horford is the next best thing after Cousins at fulfilling that criteria, though I think he only makes sense with another big free agent signing or trade.
I still want Noel on the team. Sure, he's never going to stretch the floor for you, but he's an absolute perfect for defensively for us.