The government of Canada launched John McCallum’s posting as ambassador to China with a fanfare fitting the country’s first political appointee to the world’s second-largest economy. Mr. McCallum is “very close to the Prime Minister and to me,” Foreign Affairs Minister Chrystia Freeland told a Chinese audience in August, 2017. “And he is here so that you can have a direct connection to Canada at all times.”

Now, in dramatic fashion, Canada has reverted to the way things have long been in Beijing, with a career foreign service officer as its top man in China, after Mr. McCallum was suddenly removed Saturday following a series of remarks seen as sympathetic to the Chinese position on Meng Wanzhou, the Huawei executive currently on bail in Vancouver.

Jim Nickel, 59, becomes Canada’s acting ambassador, a Saskatchewan-born diplomat described as a “steady hand at the tiller” by former colleagues. He arrived in Beijing last August as the deputy head of mission, but will now be thrust into a leadership role at a time of extraordinary tension with one of Canada’s most important economic and diplomatic partners.

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Following Ms. Meng’s arrest, Chinese authorities have detained two Canadians and sentenced to death a third, sparking a fierce row between Beijing and Ottawa. Beijing has loudly demanded Ms. Meng’s immediate release, calling the case against her a political attempt by the United States and Canada to constrain a Chinese technological powerhouse. Prosecutors in the U.S. accuse Ms. Meng of fraud related to the violation of sanctions against Iran, and have sought her extradition, and officials in both countries have said the case against her is purely a legal one.

But Mr. McCallum last week publicly described what he called strong legal arguments Ms. Meng could make against extradition, including political interference by the Trump administration. He then told StarMetro Vancouver it “would be great for Canada” if the U.S. dropped its extradition request.

In China, his removal stirred anger among those who saw the former Liberal cabinet minister as a connected and co-operative figure, a promoter of a “great new era” between the two countries and willing to say to Canadians what Beijing itself has been saying.

“McCallum was merely stating the truth when he observed that Meng has a strong case against extradition, which he rightly said was politically motivated,” China Daily wrote in an editorial posted Sunday night. “Although what he said is 100 per cent true, his words seem to have fallen on deaf ears at home. Those who had attacked McCallum should feel ashamed of themselves,” the paper added. It said “the political mess that Ottawa is floundering in could get a lot worse if it chooses to accede to the U.S. request for Meng’s extradition despite the problems with the case that McCallum, among others, has pointed out.”

Pushing Mr. McCallum from his post “is really adding salt to the injury” for Chinese authorities, said Victor Gao, vice-president of the Center for China and Globalization, a prominent Beijing-based think tank.

“It basically reveals that the Canadian authorities right now are at the end of their wits, and do not know how to effectively deal with the Meng Wanzhou situation in a very honourable and decent way.”

Canada is better served to seek peace with Beijing than to pursue extradition of Ms. Meng to the U.S., he argued. “History may actually prove that Ambassador McCallum has served the fundamental interests of the Canadian people very well, very straight-forwardly and very bravely in speaking out his mind,” he said.

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In Beijing, meanwhile, the turbulent undoing of Mr. McCallum was met with shock and concern to the diplomatic community, raising questions over whether the departure could further poison Canada’s relations with China, or whether Beijing might, as punishment, delay approval of Mr. McCallum’s permanent successor.

For some Canadians in China, however, Mr. McCallum’s departure brought a sense of cautious relief.

“Hopefully diplomats can get back to diplomacy and de-escalate a challenging situation rather than creating more additional confusion,” said Colin Bogar, vice-chair of the advisory board to the Canadian Chamber of Commerce in Shanghai.

Mr. Nickel, for his part, isn’t saying much.

On Sunday, his first full day as acting ambassador, he declined an interview request at the instruction of Ottawa, showing a caution in his public stature that had in recent days eluded Mr. McCallum.

Born in Regina, Mr. Nickel grew up in various places in Saskatchewan, where his father served in the RCMP. Mr. Nickel first came to China in 1987 to teach English at Hunan College of Tradition Chinese Medicine. The following year he moved to the capital, to the Beijing University of Posts and Telecommunications, where his teaching position afforded him a front-row seat to the student protests that erupted at Tiananmen Square. He was still in Beijing when soldiers opened fire on June 4, 1989.

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Mr. Nickel joined the foreign service in 1991, and has been posted to Tokyo, Jakarta, and New Delhi, where he was Deputy High Commissioner. Before being dispatched to Beijing, he was Director General for North America at Global Affairs Canada in Ottawa.

In temperament, he matches the classic Canadian conception of a diplomat — low-key and diligent, said friends and former colleagues.

“He is a steady hand at the tiller, with significant Asia experience and management experience at missions abroad and headquarters,” said Gordon Houlden, director of the China Institute at the University of Alberta.

He’s a “very capable guy: smart, level headed and approachable,” said Stewart Beck, the president of the Asia-Pacific Foundation who worked with Mr. Nickel for four years while he was High Commissioner to India.

And though he remains early in his posting to China, Mr. Nickel has been involved in some of the highest-profile Canadian files. He personally attended the Jan. 14 retrial of Robert Schellenberg, when the British Columbia man was sentenced to death after a single day of court proceedings.

Still, observers questioned how much difference an individual ambassador can make in a country whose leadership regularly repulses foreign interventions by telling others not to interfere in its internal business. Even when Mr. McCallum last week echoed arguments from China about the politicization of the case against Ms. Meng, Beijing responded only by reiterating its demand for her release.

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“With or without McCallum, I don’t think it makes a difference as I don’t think he had any effect on the situation,” said Michael Yen, a Canadian business consultant who has lived in China for three years.

For Mr. Yen, watching the events of recent weeks has been a dispiriting experience.

“The public threats from the Chinese government regarding Huawei and these arbitrary arrests have shown me that China isn’t an option for long-term business, nor for settling down,” he said.