So Doc will have to use Courtney Lee as the backup PG
On the plus side, Courtney Lee should be able to defend Jarrett Jack well. I don't think Jack is any quicker than Lee. Lee is taller and stronger than most PGs Jacks plays against. Lee normally does well defensively on PGs / combo guards.
On the other hand, Golden State will have a genuine and quality point guard out there on the floor. Someone who can control the tempo and dictate a game. Run his team's offense effectively. Not worried about Jack's scoring against C.Lee but Jack's passing and leadership could cause Boston some problems.