I think that for what the Celtics need, this year's first round pick is worthless. Celts need a point guard that at a minimum can be a starter on a championship team: maybe the worst starter, but a starter. Marcus Smart might be if he can pull a Grant Williams and be a 50-40-90 shooter. Josh Richardson is a good 2nd team player, but I don't think he could be a starter. Point Guards take time to develop: think of Chauncey Billups. I think Pritchard is not a good fit for this team's defense first. Maybe some super great point guard will become available in the next five years, but having White makes you more competitive while waiting to luck out on a super point guard.
The plan for the Celtics is to be a defense first team. Unlike offense, one weak link breaks a defense (see Isaiah Thomas). Looking at PG in this draft was depressing: none seemed taller than Pritchard: some were probably better passers, but not as good shooters.