If Carroll and R. Lopez are the best options, I'd prefer for the Celtics to explore trades to even out the positional depth and open up playing time for the younger guys.
Going after guys with just a year left on their deals e.g. David Lee, Roy Hibbert, Dano Gallinari would be a good way to temporarily plug some holes without compromising flexibility. Help the other guys on the team function better in their ideal roles.
Or they could just sell off some of the guys who are superfluous -- e.g. Bradley, Turner -- for younger pieces and aim to get a top 10 pick.