Chicago could also be sued, as the aviation security officers were hired by the city.
It is legal in the US for airlines to overbook flights to compensate for unfulfilled bookings.
Dr Dao's lawyer said the incident had exposed a culture in which airlines "bullied" passengers.
The Players said that back in early March, they flew to England for a family celebration using tickets they had purchased six months before for a United flight from Houston to London. "I have committed to our customers and our employees that we are going to fix what's broken so this never happens again".
Besides the damage to United's reputation, investors are nervous that airlines are planning to add too many flights, undercutting the recovery in prices. He said his client was in a "secure location" because he has been hounded by media, but that he would speak at a future date.
United does not publicly disclose how much gate attendants or supervisors are permitted to offer volunteers, but the company was broadly criticized for not offering more money to lure a volunteer. Later, Munoz offered a more emphatic mea culpa, saying: "No one should ever be mistreated this way".
She turned the experience into a movement, forming an organization that successfully lobbied for an airlines passengers bill of rights.
Munoz came under fire for praising employees after the incident and for not issuing an apology to Dao for two days.
Minnesota police department puts out hilarious 4/20 'preparations' tweet
Last year, the police department tweeted a picture of snacks underneath a trap, which also went viral. The department's tweet has been retweeted more than 50,000 times and liked more than 80,000 times.
Mr Bell said he had no plans to launch a lawsuit.
"Just because United is responsible doesn't mean the city of Chicago isn't responsible", said Demetrio. He refused, and aviation officers removed him by force.
As such, and judging from my admittedly limited and far-from-representative survey, frequent flyers seem to be a lot less outraged and surprised about what happened on that flight from Chicago to Louisville than the rest of the world is.
"This disgusting situation has provided a harsh learning experience from which we will take immediate, concrete action", said the airline on Thursday, vowing to change its rules on overbooking and booking crew, removing passengers and training its staff on how to handle such an event.
A total of 18,242 passengers were not allowed to board aircraft between April 2016 and February 2017.
The attorney was unable to say precisely how Dao was injured.
For Dao, who came to the US after fleeing Vietnam by boat in 1975 when Saigon fell, being dragged off the plane "was more horrifying and harrowing than what he experienced in leaving Vietnam", Demetrio said. Had that policy been in place on April 9, it might have averted the need to remove four passengers to make room for Republic Airline employees on their way to staff a United Express flight the next morning.
The most probable outcome from this shift in the policy governing voluntary denied boarding is likely to materialize without headline-grabbing news of passengers successfully "arb'ing" the airline.