Oscar Munoz
United Continental Holdings Inc CEO Oscar Munoz is seen in an undated handout picture courtesy of United Airlines. Reuters

United Continental Holdings Inc. Chief Executive Oscar Munoz will return to work in the first quarter of 2016, he said Thursday, three weeks after suffering a heart attack.

The news ends uncertainty about who will run United, the second-largest U.S. airline by capacity. The company had earlier appointed General Counsel Brett Hart as acting CEO and said its board had been preparing for "all potential outcomes" resulting from Munoz's hospitalization on Oct. 15.

"I am on the road to recovery," Munoz said in a statement. "My time away will be a little longer than I would like, but based upon discussion with my doctors I will be back in the first quarter."

Shares were unchanged in after-market trading following the news after closing flat at $60.30.

Many investors are expected to welcome the return of Munoz, who took over as CEO in September aiming to rebuild morale after years of strained labor relations. Munoz spent his first month on the job criss-crossing the United States to hear from employees and travelers how to improve the airline.

Hart vowed to continue this agenda in Munoz's absence. However, S&P Capital IQ cut its rating on the company's stock to "Buy" from "Strong Buy" because it believed Hart had limited experience in finance, operations or customer service.

Under Hart, the company has put forward a tentative deal for maintenance workers, paving the way for the first contract covering technicians from both United and Continental since the airlines merged in 2010.

United also said last week it would not outsource more jobs in baggage handling or customer service at least until 2017, after hearing employees were worried about job security following a round of airport outsourcing this year.