United Airlines said in a statement that CEO Oscar Munoz underwent a heart transplant surgery earlier today.

Munoz suffered a heart attack in October, five weeks after taking over from ousted former CEO Jeff Smisek, who left the company following an internal probe into actions at the airline’s hub at Newark Liberty International Airport. Former general counsel Brett Hart has been acting CEO since October 19.

In November, United said that Munoz would return to the position in early 2016. In late November he was well enough to visit workers in Chicago. His short tenure before taking ill was marked by a charm offensive to employees and an apology to passengers, a marked departure from the relationship his predecessor had with both constituencies.

United’s statement read, in full:

United Airlines today announced that Oscar Munoz, the company’s president and CEO, underwent a heart transplant operation early today and is in recovery. A transplant was considered the preferred treatment and was not the result of a setback in his recovery. The United team wishes Oscar well during his recovery, and expects him to return to United at the end of the first quarter or the beginning of the second quarter.

The company will provide additional information in the next 24 hours.

Photo Credit: United Airlines' CEO Oscar Munoz. United Airlines