McGurk's resignation, effective December 31, comes on the heels of Mattis's decision to quit the Trump administration over key disagreements with the USA president, notably the Syria withdrawal.
Army Col. Rob Manning, a Pentagon spokesman, issued a statement Sunday that did not directly address the personnel changes. After all, Beijing was identified, along with Moscow, as one of the United States' two great power rivals in the U.S. National Security Strategy and National defence Strategy.
US officials said they didn't know if Shanahan would be Trump's nominee to replace Mattis.
In a stunning resignation letter, Mattis made clear he did not see eye to eye with a president who has expressed disdain for North Atlantic Treaty Organisation and doubts about keeping troops in Asia.
The official said Mr Trump was expected to pick a nominee for defence secretary over the next couple of weeks.
Shanahan hasn't been to either warzone, and officials say he's likely to depend heavily on Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff. Dunford is scheduled to leave the job and retire September 30, and Trump has already said he is nominating Gen. Mark Milley, the Army chief, as the next chairman. Given the ongoing downward spiral in US-China relations, a new secretary of defence who shares Trump's implicit mistrust of China and his most hawkish advisers' impulses might usher in a more risky period.
"With me, hit hard instead by the Fake News Media".
Referring to the Mattis resignation, former FBI Director James Comey pungently tweeted, "To a president without any external ethical framework, folks who resign on principle must be confusing". Let's hope that we see more of that in the New Year.
"He is one of the most effective generals that we've had in many, many decades, an extraordinary leader of our time who has committed his life to his love for our country", Trump said in a December 6, 2016, speech announcing the nomination.
According to a senior administration official cited by the Washington Post, Mattis was informed of his dismissal by U.S. Secretary of State Mike Pompeo who had been directed by Trump to inform the secretary of defense.
Trump also lashed out at McGurk's resignation, casting the top diplomat as a bit player. "I DO, and it is being fixed!" he said.
Shanahan, a longtime Boeing Co. executive, was nominated for the deputy job in early 2017.
Shanahan did not serve in the military. "Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position".