I'm not sure Millsap is really the kind of guy we want to get to put next to KG, anyways.
As others have pointed out, KG is at his best when he has a big man anchoring the paint so he can float out to the elbow / perimeter. The Celtics have had some success going with two big men at either elbow, but then we're basically ceding the paint except for whatever drives we can get out of Green and Rondo.
Ideally we'd get somebody to approximate the way that KG and Shaq were able to play off one another.
Basically, KG is best when he can play as a center defensively and play like a perimeter PF offensively, and get paired with a guy who can defend power forwards and operate like a center offensively. Millsap doesn't fit the bill.