Dallas Mavericks Fans Buy Billboard Calling for GM Nico Harrison's Firing

Dallas Mavericks fans are still livid the team traded away their superstar, Luka Doncic ... and a group of fans are taking their displeasure to the streets -- 'cause TMZ Sports is told a billboard calling for general manager Nico Harrison's job is making its way around town before Wednesday's highly anticipated matchup with the Lakers.

Dallas Billboards -- in partnership with Southwest Mobile Advertising -- tells us an anonymous batch of donors reached out to them to commission the sign in opposition to the front office staffer and team owners, the Adelson/Dumont family.

The group told the companies they are "All for firing Nico and selling the team as a means of asking for accountability for the worst trade in the history of basketball."

Check it out ... the message is simple -- "Fire Nico. Sell the team."

It's not the first time a sign like this has been spotted roaming the streets outside the American Airlines Center ... as one appeared not long after the blockbuster trade went down.

While fans are not happy with Harrison or team ownership ... plenty are still flocking to the arena to see their former superstar in action.

The team announced they will be giving away shirts that say "Hvala za vse" -- which is Slovenian for "Thanks for everything."