Mayor Walter E. Washington moved quickly yesterday to appoint a temporary replacement for his general assistant, Joseph P. Yeldell, who was indicted on bribery charges Thursday. But even as the mayor delegated Yeldell's duties to someone else, uncertainty emerged over just how long Yeldell would remain off the job.

When the mayor placed Yeldell on leave with pay Thursday, his brief statement to reporters did not point out that some city lawyers believe that Yeldell can remain on administrative leave for no more than five days.

When asked by a reporter yesterday, however, Sam Eastman, the mayor's press secretary, said that the mayor understood that his action against Yeldell was only temporary. "The mayor took the action to assess the situation. I don't know what's going to happen in five days," Eastman said.

Yeldell had asked for the leave in order to have time to prepare his legal defense, which probably could not be done within five days. Several knowledgeable sources said, however, that the differences over how long Yeldell will be on leave probably could be worked out without a confrontation between Yeldell and the mayor.

Meanwhile, Washington announced the appointment of Lacy STreeter, 42, as acting general assistant. Streeter formely was an area director for the D. C. Department of Housing and Community Development in charge of the effort to rebuild the riot-scarred 14th Street NW area.

There was considerable uncertainty and disagreement yesterday about what effect, if any, the indictment of Yeldell would have, the indictment of Yeldell would have on the political standing of Mayor Washington, who is preparing to launch a campaign for reelection.

However, supporters and opponents of the mayor did agree that one almost certain result of the indictment would be to deprive the mayor of the services of his key political strategist, adviser and campaign organization architect - Yeldell.

Over the past 12 months, Yeldell has weathered a storm of accusations of mismanaging the city's Department of Human Resources and the cloud of the ongoing federal investigation of his link with millionaire businessman Dominic F. Antonelli Jr., who was indicted along with Yeldell yesterday.

The mayor, who has been a political ally of Yeldell for more than a decade, has in the last year given Yeldell increasingly more responsibility for politically important operations involving relations with community organizations, the awarding of contracts to minority firms and appointments to city boards and commissions.

As a result of that, Yeldell has emerged in recent months as a key political operative and the political alter ego of Washington.

"Joe has the best political mind in the District Building," said one Washington supporter who served on the mayor's 1974 election team. "Is there anyone that has the political judgment and has the personal respect that Joe has to take over his job? That's going to be the real fallout for the mayor."

Another confidant of the mayor, who asked not to be named, said "The kind of issues that Joe does - that the mayor put him in charge of any and says he does very well - have political overtones and political consequences for a man who's thinking about running for reelection."

Many of the mayor's opponents were reluctant to pounce on the indictment publicly, even though some acknowledged privately that it would probably reinforce allegations by the mayor's critics that Washington's administration includes many top officials not qualified to hold key jobs.

One supporter of City Council chairman and mayoral candidate Sterling Tucker said some persons would probably be embarrassed by Yeldell's indictment, which is the first against any city official, in the District's 3 1/2 year history of limited home rule.

But foes of the mayor were also quick to point out that Yeldell could receive some sympathy because he is black and many have seen criticism of him as an attack on black leadership in the city.