Rosenstein was confirmed by the Senate 94-6. Mueller reports to Rosenstein.
Since 1950 deputy Attorneys General have been confirmed by the Senate.
For example, Obama’s longest serving Deputy Attorney General: “James Michael Cole is an American attorney who served as United States Deputy Attorney General from December 29, 2010 to January 8, 2015. He was first installed as Deputy Attorney General following a recess appointment by President Barack Obama on December 29, 2010. He then was confirmed by the United States Senate in a 5542 vote on June 28, 2011.”—Wikipedia