Sources on Wall Street who follow United and who spoke with CNBC said that when they discuss the airline's "executive bench at United ... Brett Hart's name is not one that has come up."
Hart's temporary assumption to the CEO office comes less than six weeks after Munoz, who has a long track record in the railroad industry, was elevated to the top job at United after serving on its board.
Munoz was named CEO after former CEO Jeff Smisek was ousted, amid an ongoing federal criminal probe into whether the airline sought to curry favor with the chairman of the Port Authority of New York and New Jersey and get improvements made to Newark Liberty Airport by restoring a money-losing flight to South Carolina, where he has a second home.
In its announcement about Hart late Monday, United noted he "has been responsible for government and regulatory affairs, corporate real estate, customer experience, corporate security, community affairs, contact centers and food services."
"Prior to joining United in 2010, Hart served as executive vice president, general counsel and corporate secretary at Sara Lee, partner at Sonnenschein Nath & Rosenthal, and special assistant to the general counsel at the U.S. Department of Treasury."
The Wall Street Journal, in an article last week, said that when Hart was at Sara Lee in May 2010, the food company said that then-CEO Brenda Barnes was taking a medical leave, but did not reveal any details about what was wrong with her, or how long she as expected to be away. That drew complaints from some shareholders. Almost a month later, the Journal noted, Sara Lee said Barnes had had a stroke, and she left as CEO shortly afterward.
United chairman Meyer said Monday, about Hart, "Brett has taken on increasing responsibility beyond general counsel over the last few years in the operations and customer-facing areas of the company. I am confident in his ability to continue to implement the company's strategy and Oscar's mission of bringing United's people together around the shared purpose of becoming the best airline for our customers and employees."
— CNBC's Phil LeBeau contributed to this report.