Medieval priests too ill to celebrate mass

I've tried numerous Google searches, along the lines of "priests unable to say mass", "medieval priests too sick to celebrate mass", etc.  I've also looked on the Wikipedia articles for "History of medieval Christianity" and "History of Christianity", as well as "Priesthood (Catholic Church)" and "Mass (liturgy)".  I can't seem to find an answer to my very situational question.  So I'll try posting it here.  The setting is medieval Norway (just before the Black Death) in the diocese of Hamar.

What would happen if a priest fell ill and was unable to celebrate the mass?  He's not necessarily dying (though of course no one is able to know this), but his illness is such that he can't get out of bed for very long each day.  Who would say mass for him?  Would one of the local church workers, such as the sacristan for example, celebrate mass for him, or would they send to the bishop to have a replacement sent to them?  And what kind of replacement would be sent?  A newly ordained priest who wasn't given a curate yet?  Or one of the minor orders who don't have a curate at all?

Thanks to anyone who can help me with this!