Crowder isn't being traded.
He is good and cheap.
Which is why he may be traded.
We will need to get better to compete with Cleveland. That means replacing guys like Crowder in the starting lineup. He's cheap, a 14ppg scorer and a plus defender so he has value and can fit in many team systems. He may have more value to another team than the Celtics and that's why a player gets traded.
Moreover, if Fultz is drafted there's a glut at the guard spot. Thus moving Crowder gives the minutes to Jaylen Brown who may be a better fit, has massive potential and whose PER 36 numbers are quite close to Crowder's already (though not quite there as a defender yet). Thus
Crowder is a prime trade candidate - a guy we can move who has value and whose replacement is already on the roster. The only "backfill" required would be a backup SF/PF and those are fairly easy to find.
I almost expect that Crowder and either Smart or Bradley will be traded this offseason with the aim being either future picks or more likely interior help. It's time to redistribute the roster a bit to get ready to push to the next level.