Most of the concern about the Whitaker appointment has focused on what it means for the Mueller investigation, for obvious reasons. Democratic control of the House, however, effectively means that Trump can’t just make Mueller vanish into thin air. One way or another, the Russia investigation will continue until it reaches an appropriate conclusion.
Trump has made it clear that he thinks the DOJ should protect him from legal jeopardy and prosecute his enemies. His false statement about not knowing Whitaker was undoubtedly intended to send a message that Whitaker has to do his bidding to keep his job. There is no reason to doubt he will try.
The real issue here is whether Whitaker will start sacking career prosecutors and replacing them with ambitious Trump loyalists with no professional ethics, and, if he does, whether the House will do anything about it. If my worst fears on this point are realized, we are well on our way to becoming a banana republic.