I say Ron Harper Jr. He's not super young with his 26 years, but with less than 400 NBA minutes under his belt there's enough reason to assume he has unleashed potential.
If Pritchard and White would start in the backcourt, which I would not given our enormous wing depth, then the back-up (point) guard could be Harper.
I expect in first instance Conley will be given the nod, but he's completely washed and that would give Harper an opportunity for big minutes and development.
That said, I'm really curious how our wings (besides Tatum and George) will perform this year. It's a dogfight for minutes between Hauser, Scheierman, Walsh and Gonzalez.
It would make sense perhaps to trade Hauser in that regard for a back-up PG, but we know Mazzula and the Celtics demand defense reliability. There aren't a lot of point guards with good playmaking and decent defense in that price range (in the means of both salary and asset value).