Believe it or not, Lee is actually in Bradley's ballpark in terms of defensive ability
I actually do not believe this.
This past year, Bradley gave up 1.05 points per possession. Lee gave up 1.04. Advantage: Lee, though not by much
This past year, Bradley's defensive win shares was 1.6. Lee's was 2.4. Advantage: Lee, significantly
Furthermore, last year, as a team, the C's had only two lineup combinations that allowed less than .90 points per possession allowed (which is incredible). Both of them included only Lee, not Bradley. On the flip side, last year, as a team, the C's worst defensive lineup combination gave up 1.16 points per possession allowed. That lineup combination included only Bradley, not Lee.
Of course, certain conditions must be considered. Bradley obviously has a reputation as a defensive stalwart; it can be entirely possible that he would be put in whenever the opponent's star guard was on the floor. Furthermore, Lee's spot in the rotation constantly jumbled around, and it's very likely he faced the opponent's second unit far more than Bradley did. In addition, Bradley was coming off shoulder injuries last year.
Obviously, those things simply can't be accounted for. Looking at the stats though, it's clear that Lee is no joke on defense, and it isn't far fetched at all to say that he's not that far behind (or even on the level of) Bradley.