The 69-year-old grandfather suffered a "significant concussion", a broken nose for which he must undergo surgery shortly, and also lost two front teeth, his lawyer Tom Demetrio told a press conference today.
Dr Dao was removed from the plane on Sunday after he refused to give up his seat on the full flight from Chicago to Louisville.
He later said he didn't blame Dao for the "horrific event" and promised not to drag passengers off planes anymore.
United remained under a spotlight on Thursday as representatives of the carrier faced tough questioning at a city council hearing in Chicago, where the airline is headquartered and where the incident occurred.
The lawyer for a United Airlines passenger who was forcibly dragged off a flight this week said today that a lawsuit would "probably" be filed.
Lawyer Thomas Demetrio said there will "probably" be a lawsuit, but his company is now doing "due diligence".
Thomas Demetrio said at a news conference Thursday that airlines have treated us as "less than maybe we deserve".
The airline said it would "not ask law enforcement officers to remove passengers from our flights unless it is a matter of safety and security".
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The law was tabled on Thursday, and would allow adults over 18 to possess up to 30g of dried marijuana. The provinces will also decide how the drug will be distributed and sold.
A group of 21 senators also sent a letter to Munoz announcing plans to examine the incident, and New Jersey Governor Chris Christie called for the US Department of Transportation to require airlines to stop overbooking flights pending a review. The airline kicking the Kentucky doctor off to make room for four crew members.
Demetrio also said he doesn't believe Dao's race - Dao came to the US from Vietnam in 1975 during the fall of Saigon - played a role in what happened.
"For a long time airlines, United in particular, have bullied us", Demetrio stated. She said her father was catching a connecting flight in Chicago to his home in Louisville, Kentucky, after a vacation in California.
The video shined an unwanted spotlight on the airline and the little-known police force that guards Chicago's two main airports, and it could threaten the agency's future.
The Chicago Department of Aviation said three of the officers involved in the incident have been placed on administrative leave. They get less training than regular officers and can't carry firearms inside the airports. The airline's initial response was widely seen as inadequate, and in recent days United has been trying to apologize. However, Chicago Department of Aviation policy calls for its officers to not board planes to handle customer service issues, Deputy Commissioner of Security Jeff Redding said.
"What happened to my dad should have never happened to any human being regardless of the circumstances", said Dao Pepper.
Mr Zalewski also said that he is not sure if the officers have the authority to make arrests or if they are authorised only to write tickets.