I just read up on this a week or two ago: he had not one but two aneurysms, as well as a pulmonary embolism. (Only one of the aneurysms leaked, but the other still required surgery.) So, yeah, he dropped out in 1988.
(I had an aneurysm last fall so I’m recently noticing, or re-remembering, when other people have had them: Patricia Neal (who had three), for example, or much more recently, Joni Mitchell.)
(EDIT: also Larry Willis)
