Okay, here's how this plays out.
If Jaylen makes all-nba the c's will offer him the supermax, and he will sign it because nobody turns down that money. That doesn't mean he's here forever, guys force their way out all the time now, but it does take most of the downside out because it means he's not leaving for nothing.
If he misses all NBA the Celtics will offer him the max they can. Its possible that the CBA negotiations change the rules and allow the c's to offer him the regular 30% max. He MIGHT accept that, or he might elect to play next year out and try to get all-nba to secure the super max NEXT off season.
So if that's what happens, what do you do? My guess is the C's would just play it out. He still might qualify for the super max next year, and even if he doesn't that doesn't mean he's leaving. It may be he's fine staying here for the regular max but just wants to try for the supermax. I just have a hard time believing the guy is going to walk away from a contending team, where he has a pretty large offensive role, and that can offer him the most money. Usually, that combination of factors means a guy re-signs.
If the trades are some of the ones mentioned here which are like a couple role players, and a few middling picks, then honestly just keep him and try to win it all next year. I'd still lean pretty heavily on him returning as long as the c's offer the max they are able to.