United Airlines chief executive Oscar Munoz is recovering from a “heart transplant operation”, likely pushing his return to the carrier’s helm to at least the second quarter.

“A transplant was considered the preferred treatment and was not the result of a setback in his recovery,” the Chicago-based carrier says in a statement.

While United says that Munoz could return from medical leave by the early second quarter at the latest, medical professionals say such an operation typically requires months of closely monitored recovery on anti-rejection medication.

A heart transplant can even result in a patient never returning to work, they add speaking in generalities about the procedure.

Munoz has been on medical leave since suffering a heart attack in October. United’s general counsel Brett Hart has filled in for him as acting chief executive ever since.

Munoz had planned to return to work in the first quarter, he said in a letter to employees in November.

Source: Cirium Dashboard