Well, here's what I know or have seen mentioned elsewhere:
- Tobias Picker's first choice for Dolores was reported in Opera News to have been Racette (via Parterre Box).
- Racette has worked with Picker in two of his previous operas. (h/t JK for the correction here)
- She's here through October 4.
- Catherine Cook is said on Parterre Box to have done all of the stage rehearsals to date.
- Zajick withdrew from participation at the Orange Festival earlier in the summer.
So, an opera company has to make plans. Zajick's withdrawal from the Orange Festival is specifically called out in the SFO press release as having alerted the company to a possible problem. She has a history of knee problems going back a few years. Here's a 2005 Playbill item noting her withdrawal from a production back then.
My speculation: with the possibility of Zajick withdrawing, SFO started considering alternatives. As Picker's first choice for the role, Racette would have been a candidate to take over in any event. Her presence here would have made the logistics easier than otherwise. (Never mind that she's singing two roles in another opera rehearsing and playing simultaneously!) Picker would have started preparing soprano alternatives to the mezzo lines earlier this month some time, and for all we know, Racette has been working on the new role since she arrived in SF for Mefistofele rehearsals.
The thing is, we actually do not know whether SFO tried to engage a star mezzo for this role. They're not going to tell us whether there were negotiations that fell through, or how many singers or management companies they talked to. It's also good to remember that there aren't that many singers willing to jump in on a month's notice to learn demanding new-to-everybody roles and who are logistically able to fit in the rehearsal period and performances.