The imminent return of United Airlines CEO Oscar Munoz this quarter will be delayed due to his surprising heart transplant on Wednesday. While Munoz is "recovering well" after the surgery, the uncertainties about his health outlook left United employees and investors baffled and sparked concerns on the company's leadership.

Munoz's heart transplant also raised new queries on "how and when" United has disclosed information about his health condition following his heart attack on Oct. 15, just six weeks after his CEO appointment in September 2015. And since the prognosis and timetable for his full recovery remain unclear, a detailed update on his health will help ease the concerns felt by investors, who were largely left in the dark.

Following the confirmation of the transplant on Wednesday, United issued another brief statement late Thursday about Munoz's recovery progress, Wall Street Journal noted.

"[Munoz had been] progressing well, with the assistance of an implanted medical device," the company said in a statement, as per Chicago Tribune. "A transplant was considered a better alternative to long-term reliance on the implanted device and was not the result of a setback in his recovery. Munoz had been cleared to return to work prior to the transplant."

Medical experts at Northwestern Medicine, where the transplant was performed, also reassured employees and investors for Munoz's quick recovery.

"The surgical team was quite pleased with how the procedure went," Northwestern Medicine heart transplant program director Dr. Duc Pham said in the company's statement. "The patient's early course has been excellent, and the transplanted heart is functioning very well."

Northwestern Medicine Cardiac Surgery Chief Patrick McCarthy also added that given Munoz's "excellent physical condition and the rapid pace of his recovery prior to the transplant," a quick recovery and a return to his CEO duties are expected.

Munoz, 57, has been on medical leave for much of his tenure as a CEO at the world's second-largest airline. And though his leadership came during a tumultuous time at United, particularly after former chief executive Jeff Smisek's forced resignation and years of strained labor relations, Munoz made efforts to resolve the complaints of customers and employees on United Continental in recent years. He also concentrated on rebuilding the morale of the company by listening to employees and customers on how to improve the company, as previously reported.

Since early December, Munoz had been gradually resuming company-related activities, CNN Money reported. In fact, he had been collaborating with interim CEO Brett J. Hart, visiting with employees and participating in meetings. He also reportedly visited United's operations center in Chicago on Thanksgiving.

"We will, of course, be monitoring Oscar's progress closely," United's Nonexecutive Chairman Henry Meyer said. "And both his and the board's focus will be on the best interest of our shareholders."