At a news conference, Pierluisi promised to resign if the Senate doesn't confirm his nomination.
San Juan Mayor Carmen Yulin Cruz said Friday evening, hours after Pierluisi was sworn in, that her city would file a lawsuit challenging the new governor's appointment early Monday.
It's not the first time locals from Puerto Rico have rallied in support of their Island.
"If I am not ratified then the second in line, the secretary of justice of Puerto Rico, will take over the governorship", Pierluisi told reporters.
Despite opposition from the Senate, Puerto Rico's House of Representatives is expected to hold a vote on his confirmation Friday.
Rossello, who defeated Pierluisi in 2016, said the House and Senate confirmation aren't needed, citing a 1952 law.
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.
Wanda Vazquez, the secretary of justice, had been considered a serious contender for the position. She was accused of intervening in a case in which a suspect was charged with stealing government property at a home where Vázquez's daughter lived, the AP reported. "In changing the date to the month of March, we can directly confront and involve the Democratic presidential candidates with the subject of the political and economic inequality that extends across Puerto Rico".
She too is broadly unpopular and initially said she didn't want the job, but now says she would accept it. Yet, she tweeted, "The constitution dictates it". Rossello resigned under pressure after messages from him and his cabinet members were leaked.
Rivera Schatz had planned to run for governor himself next year. He also noted the challenges the island faced after Hurricanes Irma and Maria.
Others have said he still needs to be confirmed by both chambers. "I complete my mandate and the most I desire is peace and progress".