Here are my assumptions:
If IT can get on the court and play for a majority of the season plus the playoffs, and play like IT can play, this is a good trade for Cleveland. IT will nearly or perhaps even fully replace Irving's contribution plus they get Crowder who will help them. Zizic may help them some too. Then they get a really good pick. That is a good deal for Cleveland.
IF IT is out or gimpy for the majority of the season and not able to be 100% in the playoffs, this is not such a good deal for them. They seem to be planning this as if it is LeBron's last season in Cleveland. They want to be in the hunt for a title. If IT is gimpy or worse all season, they won't.
IT says he will be fine. What you can piece together from reports implies the Doctors think he is healing slower than expected but expect that he will be back on the court at full strength at some point. This is a tough call for Cleveland. How do you know. I think it is a chance they just have to take. Close the deal and hope IT is healthy soon and stays healthy. One thing I have learned about IT is it is better to not doubt or underestimate him. He has a real knack for proving doubters wrong. If Cleveland calls and asks for my advice, I would tell them to bet on IT, do the deal, make your run.