Fox News legal commentator Andrew Napolitano on Thursday said the man President Trump named as acting attorney general "does not qualify under the law" to take the job.

Napolitano's remarks come a day after Trump announced that Matthew Whitaker would take over the Justice Department as acting attorney general after Attorney General Jeff Sessions resigned from his post at Trump's request.

Whitaker will now oversee special counsel Robert Mueller's investigation into possible ties between the Trump campaign and Russia's efforts to interfere in the 2016 presidential election, the Justice Department announced.

Napolitano, on "Fox & Friends," said Whitaker is not "legally qualified" to become acting attorney general.

"There's only three ways a person can become acting attorney general," Napolitano said. "One, if you are the deputy attorney general - Rod Rosenstein - the president signs an executive order and makes you acting."

"Two is if you are already in the Department of Justice and have a job that requires Senate confirmation and you have received confirmation," Napolitano added. "That is not the case with Matt Whitaker because he's the chief of staff. That does not require Senate confirmation."

Whitaker's most recent post was chief of staff to Sessions at the Justice Department.

"Three is a recess appointment, which is not relevant here because the Senate is not in recess," Napolitano continued.

"So with deference and respect to what the president's trying to do - he has every right to have whoever he wants run the Justice Department - he has chosen someone who does not qualify under the law to be the acting attorney general," Napolitano added.

Trump for months has criticized Rosenstein, and reports emerged last month that he was planning to oust him from the Justice Department. The deputy attorney general ultimately stayed in his position, but some have raised concerns that Trump could fire him next.

Whitaker has been a sharp critic of Mueller's probe, prompting multiple Democratic lawmakers to raise concerns that he could undermine the Russia investigation.

"Given his record of threats to undermine & weaken the Russia investigation, Matthew Whitaker should recuse himself from any involvement in Mueller's investigation," House Minority Leader Nancy Pelosi (D-Calif.) said on Wednesday. "Congress must take immediate action to protect the rule of law and integrity of the investigation."