Schalke, the German Schalke player, joined the storm of anger of athletes to protest against the killing of an African-American policeman in Minneapolis, Minnesota.
The 21-year-old wore a badge on his arm bearing the phrase "Justice for George" during the match that Schalke lost to Werder Bremen at home, with an unanswered goal on Saturday in the Bundesliga competition.
Derek Chauvin, a Minneapolis police officer, was charged with murder on Friday in the death of George Floyd, 46, after mobile phone footage showed a white policeman pressing his knee to Floyd's neck, which led to a wave of violent protests.
The Schalke player followed in the footsteps of many current and former athletes, among them Basketball League winner Stephen Jackson, Los Angeles Lakers' LeBron James and Coco Jouve, the young tennis player, who also spoke of Floyd's death.
"I feel good because of my ability to use my platform to draw attention to a long-standing problem," McKinney wrote on his Twitter account after Werder Bremen match.
"We have to defend what we believe in, and I think this is the time when our voice will be heard," he added.
The 21-year-old midfielder, who grew up in Texas and joined Schalke 2016 in 2016, scored twice in 20 league games this season with Schalke 04.
McKinney participated in 19 games with the United States team and scored six goals.