Sherman Douglas?
I was thinking of Kenny Anderson. I love Kenny. I love Kenny despite being disappointed with his decline in Boston from this earlier years. As a fan though, I still loved him and rooted for him whenever he had the ball. I wished he called his own number in Boston more often. Created a bit more off the bounce both as a passer off the drive and as a scorer.
That said, I can't see Kenny or S Douglas going ahead of Cousy, Rondo or Tiny at PG. Those 3 guys have the PG spot locked down. So it would need to be a bigger guard who can play SG. I don't think Kenny or Sherman could do that.
Kenny Anderson = 5.2 assists per game 241 games
Sherman Douglas = 6.8 assists per game 268 games
Interestingly, both guys spent 5 years in Boston.