In announcing her resignation Wednesday, White House Communications Director Hope Hicks created yet another gaping hole in a West Wing senior leadership structure littered with them. But the most consequential effect of her departure may be that it deprives the White House of an aide who knows President Donald Trump better than any other colleague.

Sources in and out of the White House say that Hicks’ absence from day-to-day operations will almost assuredly further isolate Trump at a time when he has a dwindling roster of aides whom he fully trusts.

“I don’t know that anyone else can really do the job the way it needs to be done because of how the president operates,” a former Trump White House official told The Daily Beast. “Hope always seemed like the only workable solution to that. Big loss in that respect.”

The Trump White House already is operating without several key positions being filled. And in his short time in office, the president has been stung by multiple departures, including four communications directors, a chief of staff, a national security adviser, a chief strategist, and a press secretary. One GOP operative likened it to having the “B-team from year eight of an administration running things in year two” while wondering if anyone would want to fill these critical roles, considering the daily drama that such gigs entail.

Hicks’ departure may prove more significant than the others. She served alongside Trump at his business empire, on the campaign trail, and in the White House. Along that path, she earned a reputation for keeping a low profile and for her abiding loyalty to her boss.

The trust worked both ways. President Trump heavily leans on Hicks for counsel, validation, personal assistance, and comfort—to the point where her unofficial job as “Trump whisperer” typically overshadowed her official title of communications director. The president has a great deal of affection for Hicks, and has nicknamed her “Hopey.” And that affection was shared by his children as well.

“The family had come to really rely on her since the number of people there they like and trust is so small,” the former White House official said.

Within minutes of Hicks’ announcement, White House aides and those close to the president were circulating names of potential successors. Among those mentioned were White House counselor Kellyanne Conway, who the former official said is “probably the only person who could step in” and effectively replace Hicks. But, the official added, “I’d think that is unlikely” because Conway would be aiming for a more senior position or one with more influence over administration policy.

Those more seriously in the running for the communications director role, according to sources, include Mercedes Schlapp, who has served as director of strategic communications since September.

“I’d be surprised if it wasn’t Mercedes,” a Republican close to the administration said Wednesday, echoing what others in the administration had said earlier. One White House source said Schlapp was a “lock” for the job.

Should Schlapp not be the choice, there are other names in the mix, including Stephanie Grisham, who runs communications for first lady Melania Trump and previously served as a member of the Trump presidential transition team.

Schlapp and Grisham did not respond to requests for comment.

It is unclear when Hicks will formally leave her role. Her departure comes in the wake of the recent scandal involving former White House staff secretary Rob Porter, with whom Hicks had been in a romantic relationship, and on the heels of testimony before a congressional committee investigating Russian election meddling, during which Hicks admitted to telling “white lies” on her boss’ behalf.

The New York Times reported that neither of those controversies had prompted Hicks’ resignation, which had been in the works for some time. “Hope is outstanding and has done great work for the last three years,” Trump told the Times. “I will miss having her by my side but when she approached me about pursuing other opportunities, I totally understood. I am sure we will work together again in the future.”

Hicks isn’t the only communications hand exiting the administration. On Tuesday, it was made public that Josh Raffel, another top White House comms official close to Ivanka Trump and Jared Kushner, would also leave in a matter of weeks. Raffel and Hicks are similarly close and used to work together prior to the Trump era.

—with additional reporting by Sam Stein