...which we won't to be unable to find,(a solid three-point shooter + superior defender + veteran presence + inexpensive) perhaps the answer lies in finding a player with a few of these qualities as opposed to the whole package, and then relying on the team to take up the slack. Given the fact that at their core the Celtics are a defensive unit, I would suggest that the loss of 1 player, other than KG, won't change their team-defensive scheme. So let's focus on replacing, or even upgrading, the offense w/ a veteran player, smart enough to not to become a liability on the defensive end.
Suggestion. Trade Scalabrine and Powe to Charlotte for SG Matt Carroll. With our recent drafts, and the aquisition of O'Patrick, the Celts are overloaded in the frontcourt, which makes both Scal and Powe expendable. With Jason Richardson, Gerald Wallace, Adam Morrison, Derek Anderson, and Jared Dudley, Charlotte has the same problem of being overload at the wing. Charlotte would jump at this trade because, being one of the shorter team in the NBA, it would provide them much needed size at the pivot, and allow them to shed a 5 year contract that not going to see much playing time. Simultaneously, Boston gains a 6'6 27-year old, that graduated 6th all-time in scoring from Notre Dame, ranked second in scoring in the history of southeastern Pennsylvania, trailing only current Los Angeles Lakers star Kobe Bryant, and by the way, shoots a red hot, 43% from the three (that ranks 9th in the league amongst players w/ minimum 100 attempts). Carroll would be the number 1 offensive option off the bench, and a needed threat from the arc for those nights when Allen can't find the range. We can hold onto our MLE until midseason to see what other options develope..
Mainly, this trade can be done.