You can't trade Humphries after the trade deadline in this, the final year of his contract. So for an off-season trade with Humphries, you'd need a sign-and-trade.
However, if you insert Keith Bogans in place of Humphries, whose salary is non-guaranteed for next year, you can do this trade per the salary cap rules, which would allow the Celtics to effectively take back a lot more salary than they're sending out, since Portland can just release Bogans for free. I'd also note that Lee makes ~$5.5 mil next year, not $4.5 mil.
Also, if you're going to include Bradley as a sign-and-trade, you have to use Aldridge's salary for next year, which is $16 million, and not this year.
I'm not going to evaluate this trade in terms of quality, just salary cap.