It's unlikely, but there's absolutely precedent for trading a star player like this a year before they're a free agent. We had one just this year in Kyrie, even if slightly different situation given Kyrie's controversial character and decisions.
But it's actually not all that dissimilar given that I think there's going to be real questions about paying Brown the supermax after this postseason. His several major weaknesses have been on full display this postseason, and it's looking clearer and clearer that he's not a great second star fit behind JT given his strengths/weaknesses compared to Tatum, at least with our current no-true-PG dynamic. If you're not going to pay him the supermax, you absolutely have to trade him, as he almost certainly feels like he deserves it and will be slighted by not getting it.
If you can find a team that Brown would at least softly commit to for next year, you can absolutely find a trade partner with reasonable return.
The obvious choice here is Atlanta, and while I'm no Trae fan, there is some logic behind a trade package centered on a Trae for JB swap. Something like: Trae and Bey for Brown, White/Brogdon/Smart, and PP.
For Boston: our lack of a true point guard has been on full display this postseason yet again, as well as having a true alpha type star to help Tatum out in the closing moments. For all his flaws, Trae would actually fit this need perfectly. He'd provide us a true, proven point guard to take over the offense, as well as serve as a second, bonafide star to help Tatum out when things get tight down the stretch. Further, he's an excellent decision-maker and passer with a higher BBIQ. He'd also seemingly thrive with another star the caliber of Tatum.
For Atlanta: JB is from the Atlanta area and has regularly discussed loving playing back in his home area. He would seemingly relish the opportunity to go back home and play for his home team, especially given that he could be "the man" there and get all the attention. Trae has had some issues with Atlanta's coaching and management in the past, and there were already rumors about looking to move him this summer. JB would provide a similar level star who seemingly has less baggage and would be a better fit with Murray and the rest of their roster.
Not likely to happen, but I do think we have to make some sort of move for a truer point guard this summer. And moving one or more of Smart, White, and Brogdon and/or a star swap with Brown is the best way to do this.
I'm not saying you can't trade a star player, I'm saying in this situation its extremely unlikely. Ironically enough, the Irving situation is a good example of why.
In the case of Irving BRK was able to trade him, the return they got back was two low end starters and a first round pick several years down the line. That package is fine value wise, but in terms of actual competing in the near future it was a downgrade that took BRK out of contention. BRK was fine with that because they saw the writing on the wall and knew they were going to rebuild anyway. But is that the direction people want Boston to go? Obviously not, Boston is too close right now to just start over.
The reason the Irving trade got done was Dallas was willing to bet with Irving's limited market they had a strong chance of bringing him back if they offered the max they could. No such assurance is possible with the team acquiring Jaylen Brown. And it only worked for BRK because they were fine with a package they are probably going to flip for draft picks as part of a rebuild. that doesn't really work for Boston. As a general rule its hard to make a trade work when both teams want the same thing: A team acquiring Brown wants to win now, Boston wants to win now.
The Trade young trade is interesting, but I would not trade Jaylen for a small PG. Just flat no. We've tried that approach three different times (Thomas, Irving, Kemba) and the Celtics walked away from that deciding it wasn't the way to go.
And besides that I can pretty much guarantee that after a single season of watching Trae Young jack up 30 footers and play zero defense Celtics fans might literally burn down the TD garden in frustration. He is the living embodiment of everything C's fan love to complain about.