there is one scenario I have not seen expressed. What if the Celtics medical staff told the Cavs that IT is expected back by the beginning of the season, but IT told the Cavs he is not going to be back by then. This may be what is holding up the deal.
The quote for IT says it all 'Maybe I am not going to be back as soon this season as everyone wants me to be'. KEYWORD: wants
Also, keep in mind that the neither the Cavs or the Celtics are particularly interested in IT's future. Celtics dont really have a stake, and Cavs only need him to get them a championship this next season even if he compromises the rest of his career doing so. The Celtics want IT back soon to seal the trade, Cavs want him back soon to help them win. It may however be in IT's best interest to rest and heal.
IT refusing to come back sooner for his long term health is probably what is holding up the deal.