Originally Published: 12 JUN 23 10:54 ET

Updated: 12 JUN 23 12:36 ET

By Kevin Liptak, CNN

Washington (CNN) — President Joe Biden was forced to cancel his schedule Monday – including talks with NATO’s outgoing secretary general – because of an unplanned root canal.

The president first began experiencing pain in a lower premolar on Sunday. His physician, Dr. Kevin O’Connor, said a team from Walter Reed National Military Medical Center performed an exam, including x-rays, and recommended the root canal procedure. The team performed the initial part of the root canal on Sunday.

After “further discomfort this morning,” O’Connor wrote in a memo, the endodontal specialty team planned to complete the root canal procedure at the White House on Monday. O’Connor said the discomfort was expected.

The president’s team was not planning to use general anesthesia for the procedure and the 25th Amendment transferring power to the vice president was not invoked, a White House official said.

The operation is not Biden’s first root canal; when he chaired the Senate Judiciary Committee in the 1990s, he underwent middle-of-the-night procedures during Supreme Court confirmation hearings for Clarence Thomas.

Three decades later, Biden was forced to scrap a series of events Monday to allow for the dental work. That included a ceremony for college athletes on the South Lawn, which was hosted instead by Vice President Kamala Harris, and an evening reception for diplomats.

His meeting with NATO chief Jens Stoltenberg was postponed until Tuesday, according to the White House. Hovering over the sit-down will be a personnel issue: Who will replace the outgoing NATO leader when he departs his post later this year?

Biden hasn’t yet settled on a candidate to support to replace Stoltenberg, a senior US official said. The job traditionally goes to a European, but requires the backing of the American president – NATO’s largest and most powerful member.

Leaders are expected to try and coalesce around a new leader at July’s NATO summit in Lithuania, meaning Biden must make up his mind soon on who to back.

He’s already received a pitch on United Kingdom Defense Minister Ben Wallace from Prime Minister Rishi Sunak during an Oval Office meeting last week. A person familiar with the matter said Sunak entered the meeting prepared to sell Biden on Wallace, though afterward Biden told reporters he wasn’t yet convinced.

“We’re going to have to get a consensus within NATO to see that happen,” he said, calling the UK candidate “very qualified.”

A senior British official said ahead of the meeting last week that “it’s important that the next NATO secretary general carries on Stoltenberg’s good work of modernization but also understands the importance of defense spending at a critical time.”

That could be regarded as a potential knock on contenders from nations that haven’t met the NATO pledge of spending 2% of gross domestic product on defense budgets — a group that includes Danish Prime Minister Mette Frederiksen, with whom Biden met in the Oval Office last week.

Some European diplomats speculated her visit to the White House was an opportunity for Biden and his team to sound her out about the top NATO job.

Frederiksen said afterward she didn’t want to speculate about the potential of heading up the military alliance. She declined to say whether it was discussed with Biden in the Oval Office.

That hasn’t quieted speculation she may be in a leading position to earn Biden’s endorsement for the job. The alliance has never previously been led by a woman, a factor that could play into Biden’s thinking.

Other candidates for NATO secretary general could include Dutch Prime Minister Mark Rutte and Estonian Prime Minister Kaja Kallas, according to diplomats.

Stoltenberg’s term ends in October, and his spokesman has said he will leave then though his tenure has been extended three times already. He’s expected to take up a post as head of Norway’s central bank, and it’s unclear how much longer the government will keep that job open for him if he doesn’t leave NATO in the fall.

He has led the alliance through one of its most consequential periods following Russia’s invasion of Ukraine. The bloc has remained remarkably united in providing Kyiv military and economic assistance.

It’s also expanded, with Finland and Sweden both taking steps to join. The two countries have historically remained unaligned, but Russia’s aggression prompted a change of heart.

Finland’s membership was finalized in April, but Turkey has remained resistant to Sweden joining the defense alliance. Leaders hope the roadblock will be resolved ahead of the NATO summit in July.

This story has been updated with additional developments.