The transplant was the preferred method of treatment and not a setback for Munoz, United said in a statement. Munoz, 57, has been on medical leave since suffering a heart attack in October, and Brett Hart, the company’s general counsel, has run the airline in the interim.

The company, which has sought to assuage concerns that shake-ups had left an inexperienced team running its business, said it expects Munoz to come back by the end of the first quarter or beginning of the second quarter.

That suggests a potential delay from the early 2016 return that United had previously forecast.

“I’m sure investors are more worried than they were this morning,” said Adam Hackel, an analyst at Sterne Agee CRT, which cut its target for United’s share price to $70 from $78 after the news.

United said it will provide more information in the next 24 hours. The carrier’s shares were unchanged at $55.20 in after-market trading.

About 88 percent of people who have a heart transplant survive the first year following surgery, with the rate dropping to 75 percent after five years and 56 percent after 10 years, according to the National Heart, Lung and Blood Institute.

Patients often remain in the hospital recovering for one to two weeks after the transplant, the institute said. Less than 30 percent of patients return to their jobs even though most are able to resume normal activity levels, it said.

Munoz became CEO in September after the airline’s chief of five years, Jeff Smisek, resigned following a probe into the company’s relationship with the Port Authority of New York and New Jersey.

The airline has pursued Munoz’s agenda of rebuilding workers’ morale and raising customer satisfaction – lowest among rivals in J.D. Power’s 2015 ranking – by striking tentative contract deals and improving on-time performance in his absence.