Ed Macauley has his number retired (his greatest accomplishment in Boston was getting traded for Russell)
Jim Loscutoff would have his number retired but refused the honor so he got Loscy up there (mostly known for his nickname and fighting prowess)
K.C. Jones has his number retired (zero time all star scored 7.4 PPG)
So lets chill out with the idea that KG getting recognized cheapens the honor.
Those guys shouldn't be up there either. No way Boston should have 21 players retired plus Brown (1), Red (2), and Most (mic). Not that you can take anyone down, but you also don't need to keep doubling down on it. I do think you could probably remove the numbers from Brown and Red and just leave up a banner. I think there are strong arguments that Ramsey, Nelson, Sanders, and then Maxwell and DJ (for service time) should have never been up there. Maybe you do what the Lakers did for the Minneapolis players and just honor the player, but not retire the number.
The Lakers by comparison have 11 players retired, plus honor 5 players from Minneapolis (which aren't retired - well 2 are since they were also worn by LA Lakers) along with Kundla and Hearn for other contributions. That seems more appropriate.
Retiring a number should be reserved for the best of the best and in my mind KG just wasn't a Celtic for long enough to have earned that honor. You start putting KG up there, then why not have Tiny, Allen, Rondo, or Toine? Tiny in particular seems pretty close to KG. Integral member of a title team. 363 games (less than KG's 396, but not a ton). Great player before he arrived. Shorter run starting past his prime. etc.