There's a couple of questions about his appointment that make it fishy at best.
First, there's the question of why the Deputy Attorney General wouldn't take over his duties until a Senate confirmation. According to 28 USC § 508, in case of a vacancy, that's exactly what happens.
Then we have the Federal Vacancies Reform Act. To summarize, when a Senate-confirmation executive position resigns:
The assistant to the position takes over. That has not happened.
The president may appoint another person with a Senate-confirmed position to serve in that position for a limited time. That's a no on that as well.
The president may appoint another person in the agency to serve in that position for a limited time. Again, no.
I can't find a single justification for Matt Whitaker to be serving in the Attorney General position without Senate approval.