Jerusalem — The replacement last month of Israeli Defense Minister Ariel Sharon by Moshe Arens, the former Israeli ambassador to the United States, has opened the door to an Israeli accord with Lebanon.
Israel will still insist on security measures that guarantee the safety of the northern Galilee region from future terrorist attack from southern Lebanon. But the new mood in the Israeli government seems to be to accept the realistic minimum rather than hold out for the tougher maximum still advocated by Mr. Sharon, who is now minister without portfolio.
However, Israeli officials caution against undue optimism that a final Israeli-Lebanese agreement is imminent. This is so despite initial optimism that attended Foreign Minister Yitzhak Shamir's return from Washington last week, where he met with President Reagan and Secretary of State George Shultz.
A written set of American ''new ideas'' is still being studied by the Israeli government, and further clarifications are awaited from US envoy Philip C. Habib. Officials in Jerusalem insist that there is still ''a great deal more work to be done.''
In pressing for a quick ''wrap-up'' on Lebanon, the US is looking over its shoulder at Jordan's King Hussein. The King's decision on whether to join peace negotiations based on President Reagan's proposal is imminent. The decision will be based, at least in part, on the degree of progress in Lebanon.
But Israel, which has publicly opposed the Reagan proposal, is not operating under such a sense of urgency. No one here publicly links the delay to an attempt to stop the Reagan plan.
Mr. Arens told troops in Lebanon last week that while a settlement was close, patience was needed until an agreement could be worked out ''in the best possible way.''
Informed observers here say that Mr. Arens wants to minimize to the greatest extent possible the high international and growing domestic political cost of keeping Israeli troops in Lebanon. This approach - labeled ''pragmatic hawkishness'' by one observer - is reflected at the Foreign Ministry, which was overshadowed by Mr. Sharon's determined policymaking.
''The experience of the past months,'' wrote the co-opposition Labor Party daily newspaper Davar, ''has taught (Mr. Arens) what his predecessor Ariel Sharon refused to learn: that in the reality of the weak Lebanese entity and of Israeli-US relations there is no sense in striving for more than reasonable security arrangements in southern Lebanon. . . .''
Echoed Yediot Ahronot, Israel's most conservative and pro-government daily newspaper, ''Israel must act in this way because the alternative is . . . too expensive in the lives of IDF [Israeli Defense Forces] soldiers; in the suffering of our POWs and their families; in the financial burden; . . . and in the political price of remaining.''
Mr. Arens's familiarity and good relations with US officials have made him sensitive to the need for boosting sagging US-Israeli relations, and perhaps has even made some US officials overly optimistic about quick results.
But the difference in style between him and Mr. Sharon, who was openly hostile to US policy and officials, was evident during the recent contretemps over encounters between Israeli soldiers and US Marines in Lebanon. Mr. Arens picked up the phone last Friday and telephoned directly the US defense secretary , Caspar Weinberger, suggesting ways of easing the tensions.
It was Mr. Arens who suggested that Mr. Shamir visit Washington to resume direct US-Israeli consultations, which had withered during Mr. Sharon's tenure. And Mr. Arens has shut off the gushing leak of US proposals - often published on arrival by an Israeli radio correspondent close to Mr. Sharon. This infuriated the Americans.
However, despite this new atmosphere, there is still much hard bargaining to come over details for security arrangements in southern Lebanon.
While Mr. Sharon and some other ministers remain opposed, the majority of the Cabinet appears willing to concede on the key issue of a permanent Israeli presence in southern Lebanon. Israel wanted manned observation posts there. Lebanon, backed by the US, unequivocally rejected any residual Israeli troop presence, arguing that Syria would demand the same for its forces remaining in east and north Lebanon.
Israel also appears ready to be more flexible on the issue of ''mutual relations'' with Lebanon, meaning the movement of goods and persons across their border. This would probably mean that not every aspect of these relations would be immediately spelled out in a formal accord, as Israel had wished, although in practice a substantial flow of trade and tourism would probably continue.