People can talk about KG "only won one title in Boston" or wasn't here that long, but that is a very pretentious look at it in my opinion. KG absolutely changed the culture in Boston more than any other player for that 2008 title run. Pierce made changes to his game in order to make room for others, but it was KG who had his finger prints all over that title.
The way I look at it is, can I see any player ever wearing #5 again for the Celtics? For me the answer is absolutely not. I could most definitely see a player wearing #20 though (as Hayward is)
I'd even possibly consider Rondo's #9 but I don't think he had quite the impact KG did even though he was on the team longer.
Also, KG helped fetch the treasure trove of assets that landed us Tatum and Brown, so that counts for something in my opinion too. Haha