From my past experience, When a PM starts a team development, they make sure they have backup more most of the members or at least for the critical ones. As a team when we accept a project we own it and everyone tried as best as the can to hit the target on time or even sometime before to leave a buffer time for unexpected. But of course, it doesn’t work that way every time as emergencies happen. PM is always host meeting and document everything and that helps to be able to replace that member and update him with all the parts they need to know about it. My worst experience was when our CEO changed without any notice, all the members were frustrated as there was no clear direction from top mangement.