https://hoopshype.com/salaries/players/
This gives us (assuming it's accurate) a list of players by salary. It's currently set for 2021-22, but just look one column over and you can at least get a ballpark look at the list of guys making $17.1 million or less (scroll down).
Here are some names of people that might possibly help (note: they're going to be flawed players, with some real strengths and yet some real weaknesses; if they didn't have weaknesses, they'd be making a lot more money...thus, it would be an interesting decision as to which guys with which weaknesses Boston would be willing to add).
Normal Powell (16.8m) - 19.0 points, .419 from three
Duncan Robinson (16.9m) - 10.9 points, .372 from three
Malik Beasley (15.6m) - 12.1 points, .391 from three
Luke Kennard (13.7m) - 11.9 points, .449 from three
Robert Covington (12.3m) - 8.5 points, .378 from three
Danny Green (10.0m) - 5.9 points, .380 from three
Jae Crowder (10.1m) - 9.4 points, .348 from three
There are more of course. But someone like Crowder - no clue if Phoenix would move him - is maybe the kind of guy they could use. He's sturdy on defense, he's a veteran presence, he's familiar with Boston of course, and he shoots pretty well (last 2 years, .369 from three). He's not a great shooter, but he's decent enough given his size and defensive ability.
I like Kennard, as he is 6'5", and can really shoot (career .425 from three). Even with Kawhi out, he only started 13 games this year for the Clippers, and though he was 5th in minutes played, he almost always came off the bench. He's not a great defender, (113 d rating in 2021-22), but the Clips found a way to hide him on D. He's a solid playmaker and can score. Solid player for them though, but he'd probably be a great fit for Boston. No idea if they'd be willing to trade him, but they don't currently have a 2022 first round pick. Neither does Boston, so obviously the Celtics can't trade them a first rounder.
But not having a first round pick this year still eats into the roster building process for the future. Maybe Boston could deal away Nesmith (maybe the Clips think he could approximate Kennard in a year or two?) and a 2023 first round pick and pick up Kennard.