President Carter has put his administration's new assault on the energy crisis in the hands of an amiable Carter loyalist who has made his reputation, and a sizable fortune, by managing large organizations rather than acting as a policymaker or innovator.

Charles W. Duncan Jr., former president of the Coca-Cola Co. and deputy secretary of defense for the past 2 1/2 years, is not likely to create the kinds of waves at home or abroad stirred by his predecessor as energy secretary, James R. Schlesinger Jr., administration sources predicted yesterday.

Appearing at an afternoon news conference at the Pentagon, Duncan put heavy emphasis on his intention "to try to manage the Department of Energy in the best managerial style" and to "implement the policy objectives as announced by the president on Sunday night" in Carter's speech to the nation.

Duncan had let it be known within the administration in recent months that he was ready to move out of the shadow of Defense Secretary Harold Brown, who had relied on Duncan to run much of the day-to-day operations at the Pentagon. 4tDuncan had also shown particular interest in security policies toward the Middle East and Persian Gulf nations, the production heartland of the Organization of Petroleum Exporting Countries (OPEC). He participated in all high-level policy review meetings within the administration on those areas.

Some of his colleagues at the Pentagon credit a trip Duncan made to Saudi Arabia last autumn as having sparked a policy review that has brought an increase in the U. S. military presence around the Persian Gulf as a way to reassure Saudi Arabia about U. S. intentions.

Duncan said yesterday that he was convinced that the energy crisis is real. "The gasoline lines have been real, and while they are beginning to abate there is always the possibility that they will return."

He said the crisis could be resolved by achieving the president's goals on conversing energy and cutting oil imports. In response to a question, he said that nuclear power will have to continue to play a substantial role in U. S. energy production.

Duncan, 52, shied away from any policy discussion yesterday, stressing instead his role in managing DOE "to achieve the president's objectives." He said that Carter had discussed the job change with him several times over the past week and that he would begin discussing the transition with Schlesinger next week.

Duncan came to know Carter, then governor of Georgia, in Atlanta in 1970. Duncan had moved into Coca-Cola's top executive position after serving as director of the company's European operations from London for three years. He resigned from Coca-Cola in May 1974 to become chairman of Houston's Rotan Moise Finance Corp. at a salary of $205,000 a year. $ duncan is a native of Houston, with a degree from Rice University, in chemical engineering. He said yesterday that his only experience in the oil industry had been working as "a roustabout" in Texas oilfields for a few weeks.

His stock portfolio, which at one point included $14 million worth of Coca-Cola stock, might include some oil company shares, he said, but he indicated that his oil investments are not extensive.

Duncan suggested that he will cooperate fully with the new White House effort to establish Hamilton Jordan as chief of staff and decision-maker for many Cabinet matters.

"When Mr. Jordan speaks to me I will assume he speaks for the president," Duncan said, adding that he will go to the president directly on policy if he feels it necessary.

A foreshadowing of the president's personal confidence in Duncan came two months ago, when Carter send Duncan to Morocco as his personal representative to see King Hassan ii. Hassan had asked for a special envoy rather than a State Department representative to discuss U. S. Moroccan relations.

After returning from his trip last fall to the Middle East and the Persian Gulf, Duncan wrote a personal report for the president that urged Carter to take Saudi expressions of concern over security more seriously than the State Department did.

Saudi Arabia, Opec's largest producer, can be expected to greet the change at DOE with some satisfactoion. Saudi officials in Riyadh have said they were aware both of the special efforts Duncan made after his trip to get Carter to focus on Saudi security concern and of recent gloomy assessments of the Saudi royal family's chances for survival voiced privately by Schlesinger in Washington. CAPTION: Picture, Duncan: "The gasoline lines have been real . . ." AP