"If the Congress says 'We're not sure what's going to be there'" in 2018 or 2019, Slavitt said, "in all likelihood, we're going to have really significant parts of the country without coverage" from Obamacare plans.
"I think what would result from that is job loss, and so forth," said Slavitt, noting that 2 million health-care sector jobs have been created since the ACA was implemented.
The ACA has been credited with increasing the number of customers in the individual health insurance market — private plans that cover people who don't have insurance through an employer — through several provisions.
Obamacare mandated that nearly all Americans have some form of health coverage or pay a fine, barred insurers from denying coverage to sicker customers or charging them higher premiums, and offered many low- and middle-income people subsidies to reduce the cost of their monthly premiums as well as their out-of-pocket health costs.
The idea of repealing the mandate to have health coverage while continuing to bar insurers from discriminating against sicker customers has worried insurers, who would be faced with potentially fewer customers overall, but steeper benefit costs.
An estimated 20 million Americans have gained health insurance, driving the nation's uninsured rate to record lows, since the ACA went into effect. In addition to expanding coverage through private individual plans, the ACA increased the number of poor adults eligible for Medicaid, and allowed people up to age 26 to be covered by their parents' health plans.
President-elect Donald Trump has said he wants to repeal Obamacare and replace it with something better. But there is no consensus among his fellow Republicans who control Congress on what such a replacement plan will look like.
Correction: Andy Slavitt spoke on Thursday and the American Academy of Actuaries letter was on Wednesday. An earlier version misstated the days.