The loser in this is IT. If it turn out to be true that he is not able to fully play this season, he is the one that loses the most, regardless of the trade.
I think if CLE wanted to void the trade, they would have just voided it already. Many seem to disagree but I think CLE wants and needs IT to play at a high level for at least most of the season. If they don't get that out of the deal, another draft pick or swapping in Rozier doesn't really help them this coming season.
If IT can play, getting him back in Boston is not the end of the world. We were all pretty excited about the team before the trade. If IT is IT, it will be an exciting team and a good year I fully expect. If IT limps through the season or worse, that is bad for us if we get him back. As I said though, if Cleveland really believed that IT was going to limp along or worse (season ending surgery for example), the trade would already be voided.