Joe Manganiello -- Hospitalized with Very Dangerous Appendicitis

Joe Manganiello was sidelined over the last few days because of appendicitis and a surgery that was extremely serious ... TMZ has learned.

Sources connected with the actor tell us, Joe went into the hospital late last week to have his appendix removed, but when doctors went in they were alarmed. We do not conclusively know the nature of the problem, but one source says his appendix burst.

Joe, who cancelled an event Sunday, was still in the hospital as of last night.

Joe's rep has said he has a chronic condition and suggested that's why he missed his Sunday appearance. We know he ripped a bicep during shooting of 'Magic Mike' and had surgery as a result. We also know the bicep is an ongoing problem, but we're told appendicitis is what kept him away last week.