Secretary of State George P. Shultz said today that an agreement for withdrawal of Israeli forces from Lebanon is "a very doable thing," and promised to use shuttle diplomacy or "whatever is necessary" to reach such an accord.

Shultz said aboard his airplane en route here that he was embarking on his mission to revive the administration's flagging Mideast policy without any assurances from the Arab or Israeli sides on either Lebanon or the wider objective of putting President Reagan's Mideast peace plan back on course.

But Shultz emphasized that he is prepared to stay in the region "as long as there's a chance to accomplish something."

He said he will first try to break the prolonged Israeli-Lebanese stalemate to open the way for withdrawal of Israeli, Syrian and Palestine Liberation Organization forces from Lebanon.

In addition, Shultz will be seeking ways to revive Reagan's peace initiative, dealt a major blow two weeks ago when talks between Jordan's King Hussein and PLO leader Yasser Arafat collapsed.

The Reagan initiative calls for Hussein to enter expanded peace talks on the future status of the Israeli-occupied West Bank and Gaza Strip. But Hussein has said he will not come forward unless the PLO authorizes him to negotiate with Israel on behalf of the Palestinian inhabitants of these territories.

Upon arriving here, Shultz immediately plunged into a lengthy strategy meeting with Reagan's two special Middle East envoys, Philip C. Habib and Morris Draper, and the U.S. ambassadors to Israel and the principal Arab countries of the region.

On Tuesday Shultz is to meet with Egyptian President Hosni Mubarak to seek his views.

However, although Mubarak is a U.S. ally, his impact on Shultz' efforts is likely to be marginal because Egypt's peace treaty with Israel has isolated it from most of the Arab world.

Instead, U.S. officials expect Shultz' mission to move into high gear Wednesday, when he is scheduled to go to Jerusalem to begin discussions with Israeli Prime Minister Menachem Begin and his aides.

Using Jerusalem as a base, Shultz will visit Beirut toward the end of the week. He said yesterday that, depending on how things develop, he could shuttle from Israel to Amman, Jordan, and Damascus, Syria, for discussions with Hussein and Syrian President Hafez Assad. There also is a strong expectation that at some point the secretary will journey to Saudi Arabia to confer with King Fahd.

When reporters on the plane asked how long he might remain in the Middle East, Shultz replied: "I don't know yet. We just have to see how it goes."

But he said: "I'm prepared to work at it as long as there's a chance to accomplish something." And when asked if that meant willingness to engage in shuttle diplomacy, he replied: "Oh sure. Whatever is necessary."

"I'm not coming here with a preconceived American plan to try to sell to people," he said. Instead, Shultz, elaborating on an interview with The Washington Post published Sunday, said his trip had been prompted by Habib's feeling that lower-level negotiators had taken the Lebanon talks as far as possible and that "a new face" was needed to break the impasse.

"We all feel--Phil does, the president does--that it's time for me to come out and give Phil a hand," he said. Although he said Lebanon will be his first priority he acknowledged that "the peace process is very much on our minds," and he reiterated the administration's contention that Hussein has assured Reagan that he believes the Mideast initiative is still "very much alive."

Shultz conceded that the breakdown of the Hussein-Arafat talks "is certainly an obstacle to work out." But he added, "If Hussein's an insurmountable obstacle, I may just as well turn around and go back."