WEST PALM BEACH, Fla. — President Trump said in an interview that aired on Sunday that a replacement health care law was not likely to be ready until either the end of this year or in 2018, a major shift from promises by both him and Republican leaders to repeal and replace the law as soon as possible.

“Maybe it’ll take till sometime into next year, but we’re certainly going to be in the process,” Mr. Trump said during an interview with Bill O’Reilly of Fox News, after Mr. O’Reilly asked the president whether Americans could “expect a new health care plan rolled out by the Trump administration this year.”

“It statutorily takes awhile to get,” Mr. Trump said. “We’re going to be putting it in fairly soon, I think that, yes, I would like to say by the end of the year at least the rudiments but we should have something within the year and the following year.”

Mr. Trump acknowledged that replacing former President Barack Obama’s Affordable Care Act is complicated, though he reiterated his confidence that his administration could devise a plan that would work better than the law — despite having provided few details of how such a plan would work.