Tyler Zeller has been forced to undergo an emergency appendectomy after being diagnosed with acute appendicitis.
The 23-year-old had been spending time on the sidelines with an injured hip before he was hurried to hospital on the weekend for an operation.
A Cavaliers statement said: "He is now recovering at the Cleveland Clinic after the successful procedure and will undergo a period of additional recovery and re-evaluation before a timetable is established for his return to basketball activity. His status will be updated as appropriate."
Zeller is likely to be out of action for around a month with the problem, meaning that he is almost certain to miss the start of the new season.