From The Pillar:
In a passage on the role of women in the Church, Müller said that “I believe it is feasible to appoint a woman apostolic nuncio, or a woman Secretary of State or even Substitute for General Affairs.” The cardinal, who led the Congregation for the Doctrine of the Faith until 2017, made the suggestion within the context of discussing how laymen and women can occupy any Church roles not linked with the sacrament of Holy Orders.
That qualification, that lay people can serve in those positions which do not require sacramental ordination to exercise, is actually one side of a fierce theological and canonical debate which has been underway since Francis promulgated Praedicate evangelium last March.