Another reason might be that, while he is "cleared" to begin some basketball activities, he hasn't yet been given a 100% clean bill of health, perhaps due to his surgeon liking his patients to get a certain number of days past the surgery before doing so. 180 days is a typical number, but so are slightly longer numbers from 200-240. If this is the case, the C's and Green might certainly have agreed on a deal in principle, but it's being held up waiting for the final go-ahead from the doctor. The hold-up might not even be Danny (it could, but it might not), and perhaps be an insurance company.
But the Celtics and Green are pretty much wedded to each other now, because even if Green could have gotten more money, whatever team that was going to give it to him has already spent it. And the Celtics have only the bi-annual to spend whether or not they sign Green. So this deal gets done, even if we have to wait another month.