In 2015-2016, the Celtics were 4th in defensive rating and 10th in offensive rating. That has some fans thinking that the Celtics should be willing to give up some defensive efficiency and trade it for better offense. (Those fans also sometimes seem surprised that the Celtics rated so highly.)
With few free agent targets looking like a clear upgrade on offense, maybe the Celtics should try to improve the defense even more. After all, the last time the Celtics won a title, they had the best defense and only the 9th most efficient offense. The D has been surprisingly good, given the defensive short-comings of the Celtics big men as a unit. So, what if the Celtics upgraded the defense in the area of paint protection? We can quibble over which possible players would be most likely to do that, but I think that adding bigs who are defensive upgrades over Sullinger and Zeller without being complete offensive zeroes is very doable.
Upgrading the defense will lead to more opportunities in transition which will lead to better offense. The main weakness of the offense has been the relatively poor three-point shooting. This is a team that ranked near the bottom in 3P% while leading the league in layups made. There are few options out there for improving the interior defense while also improving perimeter shooting. (The idea of drafting Dragan Bender is tantalizing because he is one of the few prospects who offers that possibility. Signing Al Horford is another.)
So, in the absence of a major trade, maybe attempts to improve the roster should be aimed towards upgrading interior defense, prioritizing those players who also upgrade the offense instead of maximizing defense, while looking for three-point shooters whose defense is good enough for Stevens to be willing to play them. This would be better for the team than finding a go-to scorer who is a poor fit for the defense.