I remember being at the Rose Bowl back in 2006, when an ambulance had to come onto the field to take Ed Joyce away. It was inside the first over or two of the match, with us (England) bowling first. IIRC, SL kindly allowed Joyce's fielding replacement (Ian Bell, I think) to take Joyce's place in our batting line-up, even though technically sub fielders are not allowed to bat.
Not that whether or not Surrey can only bat with nine, ten, or eleven men is more important than the health of both players, of course.