Between a global refugee crisis, climate change, North Korea’s nuclear program, conflicts in Syria, Iraq, Afghanistan, Libya, Yemen, South Sudan and elsewhere, and a slew of internal scandals, new U.N. Secretary-General António Guterres already had some daunting global challenges on his plate. Then, he got Donald Trump.

Guterres spoke with Trump Wednesday in an “introductory call” in which they discussed “avenues for participation and cooperation,” according to a U.N. spokesperson. Last month, Trump criticized the organization following a landmark vote by the Security Council to condemn Israeli settlement building on the West Bank—a vote that was possible because the U.S. broke with precedent and did not block it. Trump tweeted:

As to the U.N., things will be different after Jan. 20th. — Donald J. Trump (@realDonaldTrump) December 23, 2016

Later he added:

The United Nations has such great potential but right now it is just a club for people to get together, talk and have a good time. So sad! — Donald J. Trump (@realDonaldTrump) December 26, 2016

In a Snapchat interview (sure, why not) after that tweet, Guterres demonstrated some grand master–level diplo-speak by noting that Trump “said the U.N. has an enormous potential. That’s exactly what I feel.”

So how bad is Trump’s relationship with the U.N. likely to get? Trump has been a consistent critic of the U.N.’s “incompetence and weakness,” particularly U.S. spending on the organization. He said during the campaign, “we get nothing out of the United Nations other than good real estate prices.” (The massive Trump World Tower is located on United Nations Plaza, which caused some tension between the developer and the U.N. back when he was building it, raising the possibility that Trump’s disdain for the U.N. is really just a long-held grudge.) He has threatened to undo several of the U.N.’s signature recent achievements, including the Paris Climate Agreement and the Security Council–backed Iran nuclear deal. His likely pick for deputy secretary of state, former U.S. ambassador to the U.N. John Bolton, openly and famously disdains the organization. Trump’s own pick for ambassador, Nikki Haley, a governor with no foreign policy experience, does not suggest he considers the position a major priority.

Guterres, who as high commissioner for refugees made a point of urging wealthy countries to do more to help migrants and refugees and has promised to do so as secretary-general as well, can’t be encouraged by Trump’s views on the crisis. And U.N. high commissioner for human rights Zeid Ra’ad al-Hussein, responding to Trump’s comments on barring Muslims from entering the U.S., deporting immigrants, and using torture, said during the campaign that Trump’s election posed a danger to global stability.

Right now, it seems likely that the Trump administration’s policies will wreak havoc on the U.N.’s recent diplomatic achievements and humanitarian agenda without pushing for the kind of institutional reforms—addressing the alarming prevalence of sexual assault in peacekeeping operations and doing more to protect whistleblowers, for example—that the organization desperately needs.

Is there any cause for optimism? Guterres, who as the former prime minister of Portugal is the first former national leader to hold the U.N.’s top job, is coming in with an ambitious reform agenda, pledging to revamp the organization’s budget rules and regulations to make it more efficient and accountable. Perhaps he could use America’s renewed threats to defund the U.N. as impetus to push these reforms through, while keeping the Trump administration engaged in, or at the very least indifferent to, the U.N.’s operations. For all the problems on his desk, this may prove to be the new secretary-general’s toughest challenge.