You don't get it, Brown's value was at it's highest when he was drafted, the more he plays and the more he comes off the bench the lower his value will get.
His value is definitely higher now then it was when he was drafted. He was not a popular pick. The point is he needs to start now, he's not going to get better without playing. You can bring along a top draft pick for a year if you're good team, maybe two but that's really pushing it.
Brown should have a major role on the Celtics this year and Tatum should have a major role on the team by next season in my opinion. You can't do that if Hayward and George are the roster.
If Brown is somehow stuck on the bench again this year, no agent is going instruct their cleints to speak or workout with the Celtics next draft (Brooklyn appears to be worse after the trade).