Despite opposition from the Senate, Puerto Rico's House of Representatives is expected to hold a vote on his confirmation Friday.
Pedro Pierluisi, a former lawyer, freely admits that despite being sworn in the Senate may not ratify him on Wednesday.
The House voted 26-21, with one abstention, to confirm Rosselló's nominee and potential successor. The secretary of state is next to line for the governor's chair when the chief executive resigns.
Jennifer González, the island's representative in the U.S. Congress, told newspaper El Nuevo Dia that Puerto Rico's constitution stated that the justice secretary should have been sworn in as governor until Pierluisi was confirmed secretary of state by the legislature.
Rosselló was due to step down at 5 p.m., a resignation he promised in response to weeks of popular protest over mismanagement, and a series of leaked chats in which he and advisers denigrated a range of Puerto Ricans.
The publication of the messages unleashed anger building for years in Puerto Rico over the island's painful bankruptcy process, ineffective hurricane recovery efforts and corruption scandals linked to a string of past governors, including Rossello's father.
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Already, Senate President Thomas Rivera Schatz has termed Pierluisi's appointment "unethical and illegal".
The departure of Rossello began a succession battle, with leaders of his ruling party opposing Pierluisi on grounds his work for the control board, known for targeting pensions and bonuses, posed conflicts of interest.
It means Puerto Rico could be engulfed by a constitutional crisis.
"If I am not confirmed in my post, then Justice Secretary Wanda Vazquez will be in line to take over as (interim) governor", Pierluisi said.
"I accept this responsibility with the greatest respect for our people and our system of government", Pierluisi said at a small swearing-in ceremony.