Larkin didn’t return because of the CBA. If the Celtics sign him to a minimum deal, there are two options:
1) A 1-year deal at the 2-year vet minimum that gave him a no-trade clause
2) A multi year deal at the 4-year vet minimum.
So either they couldn’t trade him if they needed to, or they could but he’d cost more against he luxury tax, which they’re going to be right up against.