In that event, unlikely (in my opinion) as that is, Boston's defense will take a hit, and while bringing on Brad Wanamaker ought to help, with his game yet to be tested in the NBA, it certainly wouldn't hurt to have another option in the backcourt.
Enter yet another familiar face, Avery Bradley. Poised to earn something close to a max deal at the end of his current contract, and in need of space to bring on free agent Gordon Hayward, the Celts famously flipped Bradley for Mook Morris to create just enough space for the deal to work.
Fast forward to the present, and Avery is on the mend after getting surgery to repair tears in his adductor and rectus abdominus muscles, issues which had been giving him trouble at least since the end of his tenure with the Celtics, and which became so pronounced he missed nearly half the 2017-18 NBA season, tanking his value at precisely the worst time.
There are certainly plenty of reasons why Avery might not be disposed to returning to Boston, and plenty of reasons why the Celtics might not want him, but if the sting of getting moved from a team your sweat equity helped build into a contender can be overcome, it's by another shot of reaping the fruits of that labor while also rebuilding your own value.
In other words, Boston and Bradley's interests might just be perfectly lined up.
The current lack of cap space and glut of free agents means that a lot of good guys are going to go back to their teams on qualifying offers, further shrinking available cap space and through it signing options, and most of the big-deal, top-tier free agents are going to eat up the seven or eight teams willing to use their cap in this summer's market.
What's left will be Mid-Level (MLE), Biannual (BAE), and Room Exceptions to earn money above veteran minimum deals, and none of these exceptions get above $8.6 million (for the non-taxpayer MLE). Much of this season's free agent crop will find themselves stuck with these small-money options while they ride out at least another season and possibly two more of such a cap environment, and will thus take one or two year deals with teams as a result.
Bradley's bad luck has likely put him considerably low on the list of guys teams will be seeking out to use those exceptions on, for the same reason Smart may actually end up on his qualifying offer. But at his age and with his unique skills, might conceivably get sniped by a team willing to gamble on Smart's upside outside of Brad Steven's system. At present, I see the free agency pecking order something like:
Potential max deal players
LeBron James, assuming he opts out
Paul George, same assumption
Chris Paul, though he'll likely stay with the Houston Rockets
DeMarcus Cousins, though his ruptured Achilles may cause suitors to balk
Clint Capela, who will almost certainly return to the Rockets
DeAndre Jordan, who may very well opt-in shortly to be dealt to the Dallas Mavericks
Somewhere north of average ($15 million per year) salary players
Julius Randle, who probably won't get it unless he's in a sign-and-trade
Jabari Parker, see Randle, Julius
Isaiah Thomas, see Randle, Julius and Parker, Jabari
Tyreke Evans, see, well - you know where this is going
Average salary to above non-taxpayer MLE salary players
And then, of course, Avery Bradley. A healthy, recovered Bradley would probably rate somewhere above his former teammate IT, who has himself lost significant value after hurting himself playing for Boston the season before last (I am STILL not used to saying that). Will a team want to throw some exception money his way?
Several of the above list will remove themselves from the free agency pool by not opting out or by resigning with a team they are on that already has no space, but the big fish will eat up much of space among teams which DO have it, and with more than half the league in the tax before free agency even begins, teams may be loathe to spend additional money if they don't see themselves contending.
Marcus Smart's free agency will be among the most interesting. There are teams that love him. But finding a price point high enough that Boston won't match and reasonable enough that he's not gobbling up cap space could be challenging.
This may translate to much of the "Average salary to above non-taxpayer MLE" guys finding themselves forced to take the biggest deal available, and that deal may be a bitter pill to swallow for their pride. They shouldn't take it thus, though, better basketball minds than mine have come to similar conclusions.
While this might not be how his free agency plays out at all, with Bradley's career injury history, it seems unlikely he'll go much higher in the pecking order than I have listed him, and with only about ten teams out there with more than MLE money to spend, and roughly as many more able - but not necessarily willing - to spend more than the minimum, a Boston-Bradley reunion might not end up being his worst option, or theirs.