I think that what the Celtics owners should do is to temporarily hire Leon as a staff member of the Celtics organization while he is undergoing rehab.
This way, Leon still gets paid a salary ... and he does not eat up roster space. Once rehab is complete and he is ready to play, then he can negotiate a new contract as a player.
Thia approach ia a win-win sitation for both Leon and the Celtics org.