UNESCO may be "out" in some Americans' views, but on his way in to that body, with ambassadorial rank, is expected to be Nancy Reagan's chief of staff, James S. Rosebush, according to sources close to the White House.

One of the reasons it hasn't been announced yet is that no one has quite figured out how to tell the present ambassador, former New York City attorney Jean B. Gerard, that it's time to resign.

Reports currently circulating at the White House call for Gerard to return home from Paris in September and for Rosebush to go over as soon as White House Deputy Chief of Staff Michael K. Deaver finds a suitable successor to run Mrs. Reagan's East Wing staff.

Rosebush, 33, was Deaver's selection to be Mrs. Reagan's staff director in January 1982 after Peter McCoy left to become undersecretary of commerce for travel and tourism, a position he resigned a month ago to return to California. Rosebush had been at the White House since May 1981, when he came over from the Commerce Department to help pull together the private-sector initiative program.

Although both Deaver and Nancy Reagan were known to be strong Rosebush supporters, their sentiments were not universally shared inside the White House. Some there privately regarded him as "arrogant, pompous and socially ambitious," as one insider described him.

According to another source, high-level support for Rosebush started to erode recently over a difference of opinion on how the White House promotes staff members.

Rosebush's strong suit is seen in his dealings with the corporate world. Before joining the Reagan administration, he was responsible for the $10 million corporate contributions program for Standard Oil of Ohio. At Commerce, he was director of the Office of Business Liaison. At the White House, he maintained and developed further his lines to corporations, working from within the private-sector initiative program and later from Mrs. Reagan's East Wing.

In recent months, the United States and UNESCO have been at odds over a number of issues, including UNESCO budget proposals and what the United States sees as efforts by some UNESCO members to restrict press freedom.

Gregory J. Newell, assistant secretary for International Organization Affairs at State, says of Rosebush's rumored UNESCO appointment: "I think you should let him speak for himself."

Rosebush, however, is declining that invitation.