From a Cavaliers press release:
Cavaliers forward Omri Casspi was taken to Orlando Regional Medical Center yesterday afternoon where he was diagnosed with acute appendicitis. His appendix was removed last night, with the surgery being performed by Dr. Michael L. Cheatham. He remained overnight at the Orlando Regional Medical Center and will be re-evaluated today. His status will be updated as appropriate.
Casspi, who had a strong rookie season three years ago, has struggled since and has been repeatedly discussed as desiring a trade. A recent report stated he wanted a buyout to join the Grizzlies or Rockets.