The 87-year-old Zaher Shah would probably return before March, when the national assembly is to be convened, said Zalmai Rassoul, one of his representatives in Rome.
The king, seen by many as a unifying figure for Afghanistan after three decades in exile, is sending Rassoul and Rehim Sherzoi to represent him at Saturday's ceremony in Kabul.
Karzai said his main goal as transitional leader is to "make sure that peace and security is permanent in Afghanistan."
Britain, which is leading an international peacekeeping force to help the transition, said Tuesday that its first troops could arrive in Kabul before the ceremony.
Karzai was scheduled to meet with Italian Prime Minister Silvio Berlusconi later Tuesday before seeing Zaher Shah. He plans to return to Afghanistan on Wednesday after briefing the king and getting his formal blessing.
Karzai and Zaher Shah, who are distant relatives, have a long-standing relationship. Both are Pashtun, Afghanistan's dominant ethnic group.
Zaher Shah, who has lived in Rome since his 1973 ouster, has for years tried to organize a traditional Afghan assembly, or loya jirga, to form a new government in Afghanistan.
"Before the changes in Afghanistan, I used to come here almost every month to work for the loya jirga," Karzai said before the meeting. "I used to go and pay my respects to his majesty, the former king. I've come to do the same here this time."
Rassoul said the aging monarch is heartened by recent events in Afghanistan. "There's no doubt that he's much more optimistic than in the past," he said.
The national assembly of tribal leaders that Zaher Shah will convene is supposed to choose a government to lead Afghanistan for two years while a constitution is drafted and a judicial system and police force are put into place.
Karzai was named as the head of the interim administration earlier this month during U.N.-sponsored talks in Germany on replacing the ousted Taliban regime. He was listed as part of the king's delegation to the talks.
According to the agreement, Karzai and his 29-person cabinet will rule the country for six months.
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