Ann Morrison developed the role of Lenya in readings of Lovemusik, opposite Cerveris, and I remember at the time some people being baffled that she'd been set aside in favor of Murphy. At the time, there was a stain on Murphy's reputation from Wonderful Town -- she had not done a Broadway show in three years, and this was before her rehabilitative profile in the Times, where her frequent absences and the factors that compelled her to silence were explained.
Murphy's attendance in LoveMusik was strong, but she definitely missed at least one performance, and another performance was canceled when both Murphy and Judith Blazer both fell ill, and there were not enough female covers to go around. To my knowledge, Blazer never covered Murphy in that show. I know of at least one person who saw Morrison go on and said she was superb.
I'm a big Murphy fan, but I would not say Lenya is one of her great performances -- she never seemed fully comfortable in the role or particularly well-suited to the Brecht/Weill musical style.