Mr Munoz called the incident a "system failure" and said United would reassess its procedures for seeking volunteers to give up their seats when a flight is full.
He added that the feelings of embarrassment were "palpable" for him and his United colleagues.
But when asked by ABC whether the passenger was at fault in any way, Munoz replied, "No, he can't be. We can't do that".
Dao is recovering in a Chicago hospital, said his lawyer, who is scheduled to give a press conference Thursday. It is unclear if Dao remains hospitalized.
Emirates' video asserts that they are a real airline -a highly-rated one, in fact - and mocks United's famous slogan, encouraging travelers to "fly the friendly skies... this time for real".
Paul Callan, a civil and criminal trial lawyer in NY, said the public outcry over Dao's treatment would likely push the airline to a quick and generous settlement.
All customers on Flight 3411 Sunday are receiving compensation for the cost of their tickets, United said.
The department released the following statement: "The Chicago Department of Aviation (CDA) continues reviewing the details surrounding the incident".
This news comes after the airline was reported kicking two girls off a flight for wearing leggings last month.
Munoz was widely criticized for two earlier statements, including one in which he described the passenger as "disruptive and belligerent".
By Tuesday afternoon, nearly two days after the Sunday evening events, Munoz issued another apology. These complex software packages, generally outsourced from either Hewlett Packard Enterprise or IBM, combine all major passenger service functions, including pricing, shopping, reservations, ticketing, check-in and seat assignment, said Brett Snyder, founder of the airline industry blog Crankyflier.com. They have sometimes refused airlines' requests to board planes, said spokesman and police officer Rob Pedregon.
Joya and Forest Cummings were sitting behind Dao on the United flight and began recording video after an airline supervisor asked Dao to leave the plane and he refused.
There were so many ways that the airline could have handled this situation appropriately, "but to actually drag him out of his seat and to bloody him, is just absolutely outrageous", Chu said.
Video of the incident has been shown around the world.
The man dragged from a full United Express flight is a Kentucky physician who was convicted more than a decade ago of felony charges involving his prescribing of drugs. I have to fix that, and I think that's something we can do.
Employees then contacted the Chicago Aviation Police to help remove Dao.