From The Atlantic:

Whether Mueller would actually take such a dramatic step is another question. Paul Rosenzweig, a former senior counsel on the Whitewater investigation in the 1990s, doubted that Mueller would challenge Whitaker’s appointment, “both because it is no slam dunk legally and because he is bound” as a Justice Department employee by the opinionsof the Office of Legal Counsel, which concluded in 2003 that “a Senate-confirmed position may be temporarily filled on an acting basis” by any “officer or employee” who “has served in the agency for at least 90 days in the preceding 365 days”—regardless of whether they are confirmed by the Senate. “It is also strategically incautious,” Rosenzweig said.

Still, if Mueller were to challenge Whitaker, one “solid way” to do it would be to defy him, Rosenzweig said, setting up a “live case” in which the court would have to address Whitaker’s legitimacy directly.