Just like that, one of Secretary-General Ban’s best appointees, Middle East envoy Michael Williams, has left for greener pastures, and the United Nations is left with the same old same-olds.

Over the weekend, the Guardian newspaper of Britain broke the story that, as soon as this week, Mr. Williams — a 58-year-old, silver-haired, mild-mannered gentleman — will become Prime Minister Brown’s envoy to the Middle East, among other responsibilities. Although Mr. Williams told me yesterday that “no decision has been taken,” other sources convincingly predict that his return to London is imminent.

One part of the Guardian story — and analysis in other British press outlets — was “much overdone,” as Mr. Williams put it. He is not about to become a counterweight to Tony Blair’s perceived pro-Israel bias. Unlike his U.N. predecessor, the somewhat prickly Peruvian diplomat Álvaro de Soto, Mr. Williams is well-respected in Jerusalem, where he is seen as almost untouched by traditional U.N. biases and pro-Arab tilt.

The behavior of Mr. de Soto — who recently wrote a much-publicized secret memo criticizing U.N. policies as too pro-Israel — was a lesson in Middle East diplomatic no-nos. Early on, he angered the Israeli Foreign Ministry and was then relegated to icy meetings with low-level officials. As a result, Mr. de Soto became yet another anti-Israel commentator — no position for a would-be serious player on the Middle East scene.

Since Mr. Williams’s appointment in May, he had begun to reverse that trend. His reports to the Security Council marked significant departure from the old U.N.’s tortured sense of phony “balance” between parties in the region.

Attempting even-handedness between officials of well-established state institutions in Jerusalem and representatives of ever-nascent Palestinian Arab statehood, whose constituency is unclear, is doomed to failure: The two sides are far from even. Mediation between the legitimate aspirations for independence of Prime Minister Siniora of Lebanon and the thuggery of the militias who oppose him is not only unsupported by Security Council rules, but foolhardy at that.

Mr. Williams’s reports reflected such realities. But Mr. Ban has been notoriously slow in making appointments, and his record is at best uneven. When he named Mr. Williams in May, he demoted the post from a rank of undersecretary-general to assistant secretary-general — something that, in addition to the call of his homeland, could well have figured in Mr. Williams’s decision to go to London. Although I am told that Mr. Ban has known about Mr. Williams’s imminent departure at least since his London meeting with Mr. Brown earlier this month, it is doubtful the now-vacant position will be filled quickly, and the candidate list is uninspiring.

In joining Mr. Brown’s government, meanwhile, Mr. Williams would enter a crowded field of would-be peacemakers. As the envoy of the Quartet — America, the European Union, Russia, and the United Nations — Mr. Blair is charged with building up Palestinian Arab institutions. But in Israel and the Arab world, many believe he soon will offer the mother of all Arab-Israeli peace plans.

In Jerusalem, such ideas are received with much healthy skepticism. “We’ve seen them come and go,” a government official told me recently in a conversation about Mr. Blair. “They all come with big peace plans, and then they face realities and go back home.”

No peace plan or forced mediation can solve the dispute in one fell swoop. For Mr. Blair to act according to his current job description — as a would-be Palestinian Arab nation builder — could be much more helpful than if he were to launch an ambitious new approach to bang Israeli and Arab heads together.

Fatah currently is too weak to negotiate anything. While some serious Israelis are urging behind-the-scenes cease-fire negotiations with Hamas, the organization has to change its zero-sum agenda and rid itself of the outside influence of its Syrian and Iranian benefactors before it can become a partner.

From conversations with Mr. Williams, and from reading his reports, it seems he is more aware of such realities than most outside regional mediators. Mr. Brown has done well in appointing him as an aide — and not necessarily as a competitor to Mr. Blair. For Turtle Bay, however, Mr. Williams’s departure may represent a return to a familiar, irrelevant role.