Trump's no-show could also affect negotiations.
"The U.S. will be disadvantaged when it comes to negotiating with other leaders on the EAS statement, which is likely to address issues central to Trump's agenda in Asia," said Miller, who is currently vice president of the East Asia & Pacific practice at strategy firm Albright Stonebridge Group.
That could leave Beijing with greater room to fill the leadership void as the world's second-largest economy looks to extend its sphere of influence in Asia and beyond.
A decision to forego participation in the EAS will be "disappointing," said David Adelman, former U.S. ambassador to Singapore, noting that the summit is regionally important because of its size and composition. Trump's absence could "add to the growing narrative that [his] 'America First' agenda may work at cross-purposes with traditional U.S. leadership in the region."
Tillerson's presence may be of little solace to Asian leaders seeking clarity on U.S. policy.
The commander-in-chief has frequently undercut Tillerson's diplomatic efforts so "now when Tillerson tries to conduct negotiations, people know he won't have the White House behind him," said Anne-Marie Slaughter, a former director of policy planning at the U.S. State Department.
"In the end, the White House will beat the State Department every time," she told CNBC Thursday, adding that she didn't expect Tillerson to remain in his post for the duration of Trump's term.