One thing that should be noted is Horford’s contract has no guarantee date. The first deadline for a decision is September 1st, by which he has to be stretched, but the C’s could technically release Horford in early January and still only owe the $14:5 million. Accordingly, a trade could occur in the off-season, preseason, or early regular season. Similarly, Horford could be released at the end of training camp and resigned, although that feels like something less likely to go over well with him.
But the key point is that the Celtics can decide what to do with Horford after free agency, as opposed to having to make a decision about releasing him before knowing what the alternatives are.