"You kill a leader of one of these groups and it sets them back", Pentagon chief Jim Mattis told reporters.
Abu Sayed was chosen to lead the group after Afghan and U.S. forces killed former Daesh-K leaders Abdul Hasib and Hafiz Sayed Khan in April this year and July last year respectively.
Abu Sayed's predecessor, Abdul Hasib, was killed in a military raid in April.
The presence of Sayed in Kunar is noteworthy, the other two leaders were killed in neighboring Nangarhar province the terror organization's traditional powerbase.
The local affiliate of Islamic State, sometimes known as Islamic State Khorasan (ISIS-K) after an old name for the region that includes Afghanistan, has been active since 2015, fighting the Taliban and Afghan and U.S. forces.
At the time, the United States military had said Hasib's death would "help reach our goal of destroying them in 2017". "There is no safe haven for ISIS-K in Afghanistan", said General John Nicholson, who leads US Forces-Afghanistan.