Hopefully, no sub will be needed, but I doubt they'd have Murphy fill in since she is so injury-prone and is already doing one Myrta and one Giselle already. I can't imagine they'd want to have her do 3 principal roles in one week, and the first week of the season, and risk her getting injured and then needing to recast her roles (which is already a risk; remember last year?). Stearns is really tall so I doubt Copeland would sub; same w/ Lane, although Lane/Stearns did Nutcracker last December when Lane subbed in for Seo. I think Boylston would be likely, since she's only doing one Giselle, though it's the day before. But, McKenzie rarely does what I think makes sense. 😉
And, perhaps why they had a Mother's Day sale for Giselle, which I think was a first.