Washington (CNN) The Pentagon confirmed Tuesday that President Donald Trump 's pick to be the next Secretary of Defense Mark Esper will have to step down as acting secretary once the White House formally submits his nomination to the Senate.

Navy Secretary Richard Spencer will step into the temporary role making him the third consecutive acting Defense secretary since James Mattis resigned in January.

Trump announced his intention to nominate Esper, who had been serving as Army secretary, after Patrick Shanahan's nomination dramatically imploded last month.

Esper will "cease to serve as the acting Secretary of Defense and will become solely serving as the Secretary of the Army," the Defense Department's Chief of Staff Eric Chewning told reporters at the Pentagon.

At that time Spencer "will become the acting Secretary of Defense," Chewning added, saying that "for the last two weeks we have been spending time with Secretary Spencer to get him prepared for his duties as the acting secretary of defense, that's involved a range of operational briefings from the Joint Staff."

Chewning would not say when Esper's formal nomination will happen, saying that was "under the purview" of the White House, but said that their "expectation" was that it would come "shortly."

"It's the Senate's prerogative to take as long as they need so we've prepared Secretary Spencer to remain in the role until we have a confirmed Secretary of Defense," Chewning said.

The Defense Department has not been led by a Senate confirmed defense secretary since Mattis stepped down amid disagreements over the administration's Syria Policy.

19 senior roles lack permanent appointees

Confirmation of the transition plan comes at a time when large number of senior roles at the Pentagon lack permanent appointees.

Last week the Department of Defense confirmed to CNN that 18 roles were unfilled, providing a complete list of positions currently being filled by temporary officials. The number grew to 19 on Sunday when the Trump administration's pick to be chief of naval operations, Adm. William Moran , announced his decision to retire after his judgment was questioned over a professional relationship he maintained with a former Navy public affairs official who left the service amid allegations of sexual harassment and sexual assault.

Although he is awaiting a formal White House nomination and serving in an acting status, Esper is already taking on key tasks that will lay the groundwork for how the US military will operate for months to come.

On Friday he is expected, along with the Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, to oversee a key meeting in the Pentagon's highly-secure "tank" meeting room, according to defense officials.

Friday's meeting, which is regularly scheduled, will focus on determining which air, ground and naval forces will be allocated to key areas of the world in the coming months, several officials told CNN.

While the current national defense strategy calls for the Pentagon to prioritize the challenges of China and Russia, the recent crisis with Iran has changed that effort the officials say.

US Central Command, which oversees military operations in the Middle East, had to respond to recent Iranian aggression with the deployment of an aircraft carrier, fighter and bomber jets, as well as land-based forces.

Commanders overseeing operations in the Pacific and Europe are expected to make a strong case for ensuring their areas of responsibility have the resources they want allocated.

Crucial decisions

It is not unusual for top commanders to make competing requests for forces, but Esper for the first time will find himself in the position of adjudicating disputes and making crucial decisions about force deployments.

While Spencer has been serving as Navy secretary, Chewining said, "He hasn't been exposed to the range of operational issues that the Secretary of Defense is responsible for so we spent the last two weeks getting him up to speed on what those are so he'll be ready to take on that responsibility as soon as acting secretary Esper moves out of the role."

Those briefings have been focused on "all the different operational contingencies all the worldwide panning and all the duties he will have to take on as the acting secretary."

"He hasn't been exposed to the range of operational issues that the secretary of Defense is responsible for so we spent the last two weeks getting him up to speed on what those are so he'll be ready to take on that responsibility as soon as acting secretary Esper moves out of the role," he added.

"Only one person will serve as the secretary of Defense. That person has all the authorities necessary at the appointed time to defend American interests and protect the homeland," Chewning said.

David Norquist, who is currently performing the duties of deputy secretary of Defense, will stay on in that role until the secretary of Defense nominee is confirmed. Trump is expected to formally nominate him once that happens.

The vacancies act says that an acting secretary of Defense can only serve in the job for 210 days, Chewning says that clock stops while the Senate is looking at a nominee. The deadline was to be around July 30.