The disclosure yesterday that the tumor removed from President Reagan was malignant has cast a cloud of uncertainty over the presidency, despite assurances from doctors and administration officials that Reagan is likely to resume a full and normal schedule.

"The strength of this president has always been an optimism and vigor that seems to overwhelm all obstacles," said a Reagan associate yesterday. "If he comes out of this with the same qualities, he's enhanced. If he doesn't -- well, that could be a different thing."

Reagan's reaction to the news was characteristically optimistic. White House spokesman Larry Speakes said the president still expects to leave the hospital in seven to 10 days and resume his duties "without restrictions."

Michael K. Deaver, former deputy White House chief of staff and a longtime intimate of the Reagans, said that both the president and Nancy Reagan have been "calm and realistic" about the news and added, "I don't see a major change in his attitude. Being the optimist he is, he's fully convinced that nothing will happen."

But administration officials acknowledge uncertainty about the public reaction to the statement yesterday by Dr. Steven Rosenberg, chief of surgery at the National Cancer Institute, as he launched a news conference explaining Reagan's operation.

"The president has cancer," Rosenberg said.

Two questions preoccupied the administration in the wake of this finding. The first was the day-to-day management of the presidency, in which White House chief of staff Donald T. Regan has emerged as the central figure. The second was the functioning of what might be called the symbolic presidency, at which Reagan and his managers have usually excelled.

As an example of the importance of presidential symbolism, one administration official observed that Reagan's planned August vacation at his ranch near Santa Barbara now assumes an unusual symbolic significance.

Reagan customarily spends his time on these vacations riding and clearing brush, vigorous activity for anyone recovering from a major operation. The president is expected to be still recuperating when he leaves for the ranch on Aug. 14, but his activity, or lack of it, during his vacation is likely to be a focus of commentary. The president is scheduled to remain in California through Labor Day, with his ranch time interrupted by a week in Los Angeles.

After Reagan was shot and seriously wounded in an attempt on his life March 30, 1981, the progress of his recovery astounded many people, including friends and physicians. He left George Washington Hospital 12 days after his admission, addressed Congress a month after he was shot and soon thereafter resumed a full work schedule.

In the aftermath of the shooting, Reagan aides and physicians were deliberately low-key in their predictions, but the present White House staff has been more upbeat in its predictions.

Some administration officials and associates of the president said yesterday that the White House and medical team had created such high expectations for Reagan's rapid recovery from Saturday's surgery that it would be difficult for even the vigorous 74-year-old president to fulfill them.

On Sunday, Dr. Dale Oller, who headed the surgical team that operated on the president, said Reagan was on a "spectacular post-operative course" that "surpasses, by 99.9 percent, all patients who undergo this type of surgery."

The physicians were somewhat more subdued yesterday. But both they and administration officials remained optimistic in their forecasts and at least outwardly confident that Reagan will return in full strength to his duties. Privately, however, there were a few notes of concern, if not apprehension.

"The most important thing for the president is that he get back on the job as soon as possible and be perceived as functioning fully," one official said.

Deaver, however, expressed the view that the president needed time to recuperate after coming out of major surgery and said this would be understood by the public.

In the absence of Reagan, chief of staff Regan is directing White House activities and Vice President Bush, who officiated at a swearing-in ceremony yesterday, is performing mostly ceremonial chores. Officials appeared to be deemphasizing the role of Bush, who briefly became the acting president Saturday under an order signed by Reagan just before his operation and rescinded after he came out of anesthesia.

Administration spokesmen emphasized that the president had a light schedule planned for the week and said that Bush would fill in for him at two meetings with groups of congressmen Wednesday and Thursday.

Nancy Reagan stood in for her husband yesterday afternoon at a reception for the diplomatic corps and read a message from him.