TLDR: Scoot to the Celtics makes sense for both teams.
To preface this idea, I think the Celtics have not totally closed up shop for the offseason. Dillon Mitchell remains unsigned. One would think he would be by now. Summer league is over. The Celtics are enough under the tax to sign him to spot 15 if they want, but they also have two 2-way spots available and seem to prefer that route for 2nds. Their most likely 2-way guy, John Tonje, just signed elsewhere, so it is not as if they needed to keep one open.
Anyway, I cannot figure out why Mitchell remains unsigned unless the Celtics are trying to keep some flexibility for a move. The move that requires Mitchell to remain available to sign is a 2-for-1 deal that adds payroll to the Celtics, so that they end up at 13 roster spots and less than the two-year vet minimum ($2.45 million) under the tax, but more than the rookie minimum ($1.36 million). In this instance, signing Mitchell to a rookie minimum would keep the team below the tax, and at 14 players.
Looking at the roster, the most likely traded candidate remains Sam Hauser. The Celtics could trade him and bring back roughly $12.5 million while staying under the tax. However, if they added either Garza or Scheierman to a deal, they could bring back roughly $13.9 million, and have room to sign Mitchell and stay at 14 players and under the tax.
It is a short list of players who make between $12.5 million and $13.9 million, and are eligible to be traded (because they are not rookie 1st round picks or free agency signings):
Risacher
Scoot Henderson
Lillard
Avdija
Stephen Adams
Dylan Harper (I can dream, but obviously not)
Donte DiVincenzo
Of that small list, Avdija and Harper are clearly unavailable, DiVincenzo is out for most or all of the year on an expiring, and Rishacher was just traded. That leaves Lillard, Adams, and Scoot.
Lookig at the Blazers roster, they have 5 PGs, and Scoot is probably fourth on that list behind Morant, Jrue, and Lillard. They also have 5 centers. Meanwhile, they have 3 playable wings in a league where teams regularly play three wings at a time. Accordingly, the following trade may make sense for both teams:
Blazers receive:
Hauser
Scheierman
Celtics receive:
Scoot Henderson
This gives the Blazers two more rotation wings. It is not an awesome return for the Blazers on a former #3 pick, but it is comparable to the Risacher-Dort trade of last week, and arguably better given the respective contracts of Hauser and Dort. Scoot has shown flashes of promise as a distributor and on defense, but he is not likely to be extended this summer by Portland, and are they really going to have a restricted free agency battle over their 4th-string PG?
For the Celtics, Scoot could become PG #3, and Pritchard could move to the starting lineup. They would still have plenty of wing depth in Gonzalez, Walsh, and Harper Jr, and with Mitchell as an emergency option. The Celtics could try to extend Scoot, and could go the low-$20 million range next summer to keep him in restricted free agency if they did not while staying under the 2nd apron (this would probably be enough to fight off most offer sheets). It would be a high-upside (he was #3 just three seasons ago, if he stays healthy he could pop), moderate risk (the Celtics could have given up two 40% 3-point shooters on good contracts the next 2-3 seasons for an expensive 3rd string PG who flames out).