If evergreen resigns today, a special election will be held next weekend. The governor can appoint a temporary replacement. He or she must be from the same party as evergreen, though, IIRC, evergreen was NNP at the time of the election and it is no longer a federally recognized party. I'll double check the registration.
If not, we'd have to wait until evergreen misses the requisite number of votes before removal. I don't think you can take an indefinite leave of absence that might last the entire term.
Get well, evergreen! That's the most important thing.
Regardless of whether the NNP is federally recognised (and I don't think we are) - we only have three members in the Northeast: Oakvale (who's Senator), myself and Evergreen. There'd be no-one else to fill that role, unless someone else joined the NNP without me noticing.