Some players, while good in a general counting-stats sense, tend to not elevate the performance of their teammates.
Consider how good some of Russell Westbrook's stats were, especially all of the triple-doubles, yet a lot of his numbers were "hollow," as he didn't seem to really create more wins for his teams. I think Chris Paul was sorta like that, too.
Well, it seems that Brad views Jaylen this way. Since the trade, I've been doing a lot of reading on Jaylen's numbers, and though I'm far from an expert, this is how his stats come across to me as well.
Combine that with the fact that keeping him around would result in two players eating a huge part of the cap space, and I see why Brad traded him. The trade shocked me -- that it was with a division rival, and the return seemed underwhelming -- but it makes sense to me now. And since there are certain things about Brown that rub me the wrong way, I'm not sad that he's no longer a Celtic.
How good the return is remains to be seen, as does the outcome of the upcoming season, but I think this trade could ultimately shake out as being a good thing, especially if it enables the Celtics to acquire one or two high-impact (non-empty-stats) players over the next couple of seasons.