A few other things to consider about some possible reasoning for this trade.
1. Not sure if there is any truth to this, but oen rumour I saw going around was that the reason the Giannis trade fell through is that the Bucks wanted Hugo and the Brad would not include him. Brad Stevens has been one of the greatest I've ever seen so far seeing potential where nobody else seemed to, so so if he actually is that high on Hugo then he might think the kid has potential to one day replace JB as a really high iimpact starter.
2. While Tatum was out last year and the Celtics only had one ISO heavy scorer (in Brown), the ball moved a lot better, and the less celebrated players on the roster (like Queta, Hugo. Walsh, Garza., Sceierman) seemed to really flourish with those oportunities. Brad had young guys on extremely cap friendly contracts going out there and making a mockery of the analytics data - and the crazy thing is that in the games where Brown sat out, the Celtics didn't necessarily see a huge drop off as many might have expected.
Once Tatum started getting back to his old self, the Celtics largely fell back to their old tricks - where everyone else on the team mostly stood around and watched while Tatum and Brown played "my shot, your shot". It's not only last year either. Since Tatum and Brown have been in Boston together the Celtics are:
* 90-36 (0.714 - equivalent to a 58 win season) in games where only Tatum played
* 72-45 (0.615 - equivalent to a 50 win season) in games where only Brown played
* 332-167 (0.665 - eqivalent to 54 wins) when both have played
That's a fairly large 243 game sample size in which the Celtics have had only one of the Jays on the court, and the difference in win record is really not dramatic - in fact their win record with only Tatum is actually HIGHER then it is when both have played.
We all know Brad is a big analytics guy, so if you look at this data and also watched how well the Celtics played as a team while Tatum was out, he may well have come to the determination that having Tatum and Brown together is actually a negative rather than a positive and that trading one of them away is actually an "addition through subtraction" move.
The Celtics could lock themselves into paying $120M+ per season with the Jays over the next 4-5 years, or they could trade Brown away, bring in a guy like Paul George (who at his age is likely quite happy to accept a support role behind Tatum) and then have a bunch of extra cap space in a couple of years to go after a player(s) who better compliment Tatum.
In addition Brad also adds more draft picks to their existing stash. Next year PG will be down to one guaranteed year on his contract, and Brad might be able to offer one of those picks to another team to take that contract - leaving Boston with a huge TPE that they could then use to bring in a big impact player. It also gives them a big contract, which they could potentially use to match salaries for a max salary player in a future deal. It might give the Celtics more flexibliity then we realise, and in the meantime George is a productive veteran who can contribute to team.