The league has confirmed 14 cases of COVID-19 since it suspended the season last week.
Mitchell became the second NBA player to test positive for COVID-19 after teammate Rudy Gobert last week.
The NBA has suspended the season until further notice following news that the Utah Jazz center had tested positive.
"As a white man, I have to hold my fellow white men accountable," the NBA veteran wrote in The Players' Tribune.
Westbrook did not apologize for threatening to "f*** up" the fans during the exchange.
The guard had been arguing with officials.
Tanner Ainge, the son of Celtics general manager Danny Ainge, is running for Congress in Utah and had encouraged NBA star Gordon Hayward to stay with the Utah Jazz.
Miami, Boston and Utah are all in the mix for his signature.
Green also won Defensive Player of the Year.