FC Dallas owners have condemned the "repulsive and unacceptable" racist comments and death threats sent to defender Reggie Cannon.
Cannon criticised fans for booing FC Dallas and Nashville players when they took the knee before their Major League Soccer (MLS) match on Wednesday.
"There is no place in our sport, or in our country, for the kind of horrific vitriol Reggie has had to endure," FC Dallas said in a statement.
"Violent threats are never warranted."
Almost 3,000 fans were allowed inside the Toyota Stadium under local coronavirus rules, and some boos could be heard as players knelt during the national anthem to protest against racial injustice.
Cannon said afterwards that the boos were "disgusting" and "disgraceful".
"We want to be clear: we love and support Reggie Cannon," FC chairman Clark Hunt and president Dan Hunt said.
"We will continue to work together - with our players, our coaches and our fans - against racism and in the cause of equality for all."
MLS said comments made on social media after the match were "appalling".
A statement added: "MLS will not tolerate any abuse or threats to any individual player or team who decides to exercise their right to peaceful protest."
There have been worldwide protests since the death of George Floyd, an unarmed African-American man who died on 25 May after a white police officer knelt on his neck for several minutes.
Former Minneapolis police officer Derek Chauvin faces charges of second-degree murder and manslaughter, while three other officers were also sacked and charged with aiding and abetting murder.