Almost three months after suffering a heart attack on Oct. 15, United Airlines' parent company United Continental Holdings Inc.'s Chief Executive Officer Oscar Munoz underwent a heart transplant on Wednesday. While his return was previously reported in the first quarter of 2016, the latest report said his imminent comeback will be by the end of the first quarter or the start of the second quarter.

Munoz was hired on Sept. 8 as United's CEO but he went on sick leave after suffering a heart attack in October. According to Bloomberg Business, the 57-year-old executive was temporarily replaced by the airline's general counsel, Brett J. Hart as the company's acting CEO. Before his medical leave, Munoz had vowed to improve customer relations at United. The company also had a series of positive developments with its unions, including a potential contract extension with its pilots union and a tentative contract agreement with the Teamsters-represented mechanics.

"A transplant was considered the preferred treatment and was not the result of a setback in his recovery," the Chicago-based company said in a statement.

The latest news on Munoz's health, however, have also ignited some concerns to the investors as United continues to assuage concerns regarding the company's management shakeups.

"I'm sure investors are more worried than they were this morning," Sterne Agee CRT analyst Adam Hackel said, as per Reuters.

Meanwhile, 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.

Munoz became United's CEO during a tumultuous time at United and after the forced resignation of Jeff Smisek, the airline's chief executive officer for five years. Smisek left the company amid a federal probe into the airline's scandalous connection to the former Port Authority of New York and New Jersey chairman. The incident remains a part of an ongoing federal investigation, CNN Money reported.

Under Munoz's leadership, he made efforts to resolve the complaints of customers and employees on United Continental in recent years, which surfaced after the merger of United and Continental airlines. He had also been focusing on rebuilding the morale of the company following years of strained labor relations.

Before his hospitalization, Munoz has been traversing the United States to hear from employees and customers on how to improve the company. And to express his gratitude, he wrote a letter to his employees last year.

"Thank you for all the thoughts and prayers you sent to my family and me over the past few weeks," Munoz wrote. "I'm personally grateful to Brett and the entire management team for their leadership and for embracing United's new direction... While I have been part of the United family for only a short time, I've built many valued and strong relationships so far, and I can't wait to come back."

"As I'm sure you can appreciate, this has been a difficult time for me and my family," Munoz added. "Your messages of encouragement have helped profoundly. I look forward to seeing you in 2016 to continue working together to build a great airline."