Defense Secretary Jim Mattis will hand over the reins of the Pentagon to Deputy Defense Secretary Patrick Shanahan as acting secretary of defense during a phone call scheduled for Monday.

Beginning January 1, Shanahan — who has kept a low profile during his tenure, will assume command of the Department of Defense, replacing Mattis, who recently resigned over the president’s decision to withdraw U.S. troops from Syria.

President Trump has personally endorsed Shanahan, announcing December 23 in a tweet:

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great! — Donald J. Trump (@realDonaldTrump) December 23, 2018

During a visit to Iraq, Trump told reporters that Shanahan, 56, could be leading the Defense Department for “a long time.”

“He could be there for a long time, I’m in no rush,” he told reporters last week.

The knee-jerk criticism of Shanahan has been that he lacked military experience and foreign policy experience before coming to the Pentagon in July 2017. However, his supporters argue he is well-equipped for the job.

They note he has worked closely with Mattis for the last 17 months, often filling in at meetings, conference calls, and at the White House when Mattis traveled. Defense officials say he has been in virtually every meeting, engagement, and discussion along with Mattis.

Supporters also point to Shanahan’s experience in the defense and aviation industry for more than 30 years — something that will come in handy as the Pentagon looks to modernize the force as well as cut costs on pricey weapons systems. He last served as senior vice president of supply chain and operations at Boeing, overseeing the company’s manufacturing operations.

Shanahan is known among aviation experts as a problem-solver, and is widely credited with almost single-handedly saving Boeing’s 787 Dreamliner.

“I like making things work, and work well. I’m an industrialist. I’ve spent my life building large, complex machines with complicated supply chains at scale,” he said September at the Air Force Association conference. “I’m product-driven. I’m focused on performance, and I’m focused on making change at scale.”

The Air Current’s Jon Ostrower, who has covered him at Boeing for nearly a decade, recently wrote, “A renowned fix-it guy, Shanahan is a tactical genius as a process engineer. He’s got an extremely low tolerance for bulls***.”

As deputy defense secretary, Shanahan has managed the Pentagon’s day-to-day operations, considering himself the “COO,” or chief operations officer, of the department.

A defense official said on background, “When you look at what he’s done at Boeing, he’s broken down problems into consumable portions, then developed a strategy, generated alignment, built systems, and generated change at scale.”

“It’s the same approach he’ll have here. I think people feel that being Secretary of Defense involves directing operations. It’s about moving the Department toward a specific direction,” the official said.

Some journalists have suggested that Shanahan’s ties to Boeing would result in lucrative contracts for the company. NPR’s Ari Shapiro said recently that Shanahan’s expected focus on modernizing the U.S. military — also a stated priority for the most recent defense secretaries — “could mean more money for Boeing.”

According to defense officials, Shanahan has agreed to recuse himself for the duration of his service in the Department of Defense from participating in matters in which Boeing Company is a party, including contracts, decisions, appraisals, or evaluations. Any matter involving Boeing will be routed to one of several designated appointees.

And Trump sees Shanahan’s private sector experience as an asset — particularly as he looks to cut down on waste in the Pentagon’s budget.

“Our friend Shanahan is a good man. He’s done a great job. He’s a good buyer. I wanted somebody that could buy. Because I’m giving them a lot of money and I don’t want it to be wasted,” Trump told reporters during his trip to Iraq.

“Shanahan was at Boeing. Did a great at Boeing. Was there for a long time. Boeing is a hell of a company. He did a great job. Very responsible for the success for a certain plane — the Dreamliner. He’s a respected man.”

Shanahan also has a science and business educational background, that will also help him with the nitty-gritty of modernization and acquisition reform. He has a Bachelor’s degree in mechanical engineering from the University of Washington, a Master’s in mechanical engineering from the Massachusetts Institute of Technology, and an Master’s of Business Administration from MIT’s Sloan School of Management.

Shanahan has also taken on a weighty portfolio during his time as deputy defense secretary. He has spearheaded turning the president’s desire for a Space Force into a reality, which has helped him develop a good working relationship with the White House and vice president.

He also helped craft the National Defense Strategy formulated under Mattis, which shifted the Pentagon’s focus from terrorism to great power competition with China and Russia. He has also been responsible for making sure the military was implementing it.

There is also sense within the Pentagon that Shanahan will represent continuity from Mattis.

“Mr. Shanahan will remain focused on the National Defense Strategy. He was instrumental in crafting the document. His priorities are in there, so they have not changed,” said Shanahan’s spokesman Army Lt. Col. Joe Buccino.

The defense official said the impression that Shanahan has been siloed off working on special projects during his tenure is not true. “That’s not the case — he’s had everything.”

Shanahan is also a contender to take over the role permanently, according to reports. He was confirmed by the Senate as deputy defense secretary in July 2017 by a 92-7 vote.

However, Shanahan will face some near-term tests. He is expected to testify on the Trump administration’s 2020 defense budget request to Congress in February and will face questions from hawkish Republican senators and Democrats who are skeptical — if not downright hostile — to Trump’s policies.

Shanahan will also have to navigate a relationship with the press that had at times gotten prickly under Mattis. The retired four-star general was criticized in the mainstream media for not holding enough televised press conferences.

Shanahan has already had several public engagements with reporters, including in October at the Military Reporters and Editors conference where he fielded questions from dozens of reporters on camera.

In light of Mattis’ revered stature, Shanahan will have big shoes to fill, but so far, he has faced government service with enthusiasm.

“I’m often asked about the transition to the department with an undertone, ‘Is it what you expected?’” he said at the September conference. “I tell people it’s like breaking up with your longtime girlfriend and finding the love of your life.”

Shanahan grew up in Laurelhurst, a suburb of Seattle, Washington, according to the Seattle Times. His father, Mike Shanahan, was a Vietnam veteran and Bronze Star recipient, and later served in law enforcement for over 25 years.

Shanahan keeps a framed photo of his father in his office.

“He taught his boys to love their country and value its freedoms,” he said at his confirmation hearing last year. “He taught us to treat people fairly, respect law and order, and the importance of protecting the community.”