Senator Mitch McConnell was hospitalized about a month ago, so Kentucky has had only 1 of 2 senators representing the state in the U.S. Senate. The people of the state deserve full representation.
If his doctors are certain he will return very soon, I would think most people would find the absence acceptable but let the people know (or at least let the governor K\know).
If his doctors think he will not be able to return, I would think it appropriate to resign.
If the doctors are uncertain and the law and Senate rules permit, perhaps the governor can appoint a temporary replacement who can serve until Senator McConnell returns.
In any case, each branch of Congress should set up some rules or guidelines for this type of situation.