The roster will probably look very different in a few months. We don't yet know if Danny is committing to the youth movement, or if he has some moves in mind to keep us competitive. So we'll see.
If we're going young, then Williams probably backs up Baynes while playing 20-25 minutes per night. If we still expect to make deep playoff runs, then I doubt Williams will be on the team on opening day. He's probably one of our most tradeable assets (enough value to entice other teams, not enough for the Celtics to lose sleep over giving him up).