The 14-year NBA veteran sounded off on social media on Monday, citing owner Robert Sarver as the organization's main issue while highlighting his concerns about the long-term future of star guard Devin Booker.
"Coaching isn't the problem with the Suns, the owner is!" Barnes wrote. "Sarver is the worst owner in the NBA. Do that great city of Phoenix a solid and sell the team, they deserve a winner! You're wasting Devin Booker's career."
Kokoskov was Booker's fourth head coach in as many seasons and the team's sixth over ten years. The Suns haven't exceeded 24 wins since Booker's rookie 2015-16 campaign, and they finished this past season with the NBA's second-worst record at 19-63.
Booker averaged career highs in points (26.6), assists (6.8), and field goal percentage (46.7 percent) during the 2018-19 season.
In March 2017, the former Kentucky product dropped 70 points against the Boston Celtics, becoming the youngest player ever to score 60-plus points in a game. Just over two years later, Booker became the youngest player to post back-to-back 50-point games.
The 22-year-old agreed to a five-year, $158-million contract extension in July 2018 that kicks in next season.