The Wall Street Journal reported that Munoz suffered a heart attack Thursday and is being treated at a Chicago hospital.
United Continental, the parent of United Airlines, acknowledged that Munoz was hospitalised but did not explain why.
"We have been informed by Oscar's family that he was admitted to the hospital on Thursday and we will provide further details as appropriate," the company said in a brief statement.
"In the meantime, we are continuing to operate normally. Our thoughts and prayers are with his family and we are respecting their privacy."
The company immediately replaced Smisek with Munoz, president and chief operating officer of CSX Corporation, a US rail operator.
News of Munoz's illness sent United shares plunging 4.5 percent to USD 55.15 in afternoon trade on the New York Stock Exchange.
