Not since Ponselle?

The wonderful Karita Mattila isn't singing tomorrow's Met matinee - and thus broadcast - of Fidelio; instead, her cover, Erika Sunnegardh is singing. She is 40 and making her Met debut (!); she's also under contract to cover Mattila's Elsa, and she's taking part of a run of Turandot a season or so down the road. I'm rather astonished to find this in the Times article about tomorrow; the quotation is from Joe Volpe's not-yet-published memoir:

"Not since Rosa Ponselle's debut in 1918, opposite Caruso in 'La Forza del Destino,' has the Met given an unknown singer such an opportunity," Mr. Volpe wrote.

I guess that's about her upcoming starring role and the covers, but I wonder how that plays in the context of Astrid Varnay's 1941 debut, covering Sieglinde for an indisposed Lotte Lehmann, followed by singing Bruennhilde the following week covering for Helen Traubel.