For the first time in years the United States is on the sidelines during an upheaval in Middle Eastern diplomacy, operating as a courier between Egypt and Israel in a venture that can either confound or reinforce American strategy.

Carter administration planners said that none of the developments yesterday, in Syria or in Egypt, altered their basic assessment that Egyptian President Anwar Sadat seeks to enhance a Geneva conference, not replace it with an Egyptian-Israeli settlement.

By administration calculations, if Sadat is at all successful in his precedent-shattering trip to Israel this weekend, the most probable outcome is the acceleration of Egyptian-Israeli negotiations within the framework of a broad Arab-Israeli conference in Geneva, Syrian President Hafez Assad's challenging attitude toward such a conference, however, now has been underscored by his open disagreement with Sadat, the leading Arab champion of the American Geneva conference formula.

President Carter said Assad's opposition to Sadat's bold venture into Israel cannot be considered surprising.

"Predictably, President Assad did not endorse" the trip, Carter said.

Carter, speaking briefly with reporters as he planted a tree on the White House lawn, was asked whether he thought Sadat's Israeli trip might backfire against him. "There's always a danger," Carter replied, "but it's a good thing. I'm in a favor of it." The trip, he said, "has good potential."

Israeli Prime Minister Menahem Begin telephoned the President yesterday to talk about "the significance, both symbolic and substantive," of the Sadat visit to Israel, White House spokesman Jody Powell said.

Begin telephone about 3.50 p.m., Powell said, and the two leaders talked for about 10 minutes.

The White House released a message later from Begin, thanking Carter for encouraging the climate for the visit.

As released, the message said, "I believe, Mr. President, that without your contribution these events could not and would not have been set in motion. You have created it. Mr. President, and I express to you my deepest, heartfelt thanks."

This was not intended to imply that the United States initiated the Sadat trip, White House officials said. Carter already has said the administration had no forewarning of it. The administration has been anxious, however, to align itself with the mission, once it was apparent that Sadat would make it.

Administration officials avoided any public comment yesterday on the resignation of Egyptian Deputy Prime Minister and Foreign Minister Ismail Fahmi, or the refusal of Egyptian Minister of State Mohammed Riad to replace Fahmi.

Fahmi has been a key figure in American-Egyptian diplomacy for four years as Sadat's minister, "the tester," as one senior official expressed it, for many Sadat initiatives.

"There never was any doubt, however, about who was running Egyptian foreign policy," the U.S. official said, for that was "always Sadat," often directly with the United States.

Differences between Fahmi's caution as a professional diplomat and technician, and Sadat's boldness and imaginativeness, have been frequently evident. During the September-October negotiations at the United Nations between Secretary of State Cyrus R. Vance and Arab-Israeli diplomats, it was apparent even to reporters that Fahmi wanted to move more slowly than Sadat through the procedural obstacles to a new Geneva conference.

The defection of Fahmi from Sadat over the Israeli trip is obviously troublesome for Sadat, American sources agreed, showing that even some of his closest advisers consider Sadat's decision "a bad move." But if Sadat "pulls it off, he's a great hero," one U.S. official noted.