Grant Hill.
He could definitely hold the fort and run our plays. Plus, he has a lot to offer aside from playmaking like taking the outside shots (that we still want Rondo to take), rebounding, defending PGs, hustle and running the break. He will be very tall for a PG and that could cause a lot of mismatches against teams. He could also sub for the SG or SF if needed. Ron Harper did well as PG for LAL that I don't see that we can't do similarly for the Celts.