Golf Channel's Rex Hoggard reported the news:
Per the event's official website, Koepka qualified as the team's top-ranked player. He made his Presidents Cup debut in 2017, going 2-2-0 in a 19-11 win for the Americans.
The 29-year-old regrets not being able to represent his country:
"Today, I am announcing my withdrawal from the U.S. Presidents Cup Team because of my knee injury. I notified Captain Tiger Woods that despite constant medical care and rehab, I am not able to play golf at this time. I consider it to be a high honor to be part of the 2019 team, and I regret not being able to compete.
"Since my injury in Korea, I have been in constant contact with Tiger and assured him that I was making every effort to be 100 percent in time for the Presidents Cup in Australia. However, I need more time to heal.
"I'm sorry I won't be able to represent the Red, White and Blue this time around, and I wish my teammates nothing but the best as they work to retain the Presidents Cup for the USA."
Fowler is a strong replacement, having played in two Presidents Cups already. He went 3-0-1 in 2017 and finished 11th in the standings this year, with the top eight making the team.
He's excited to get to work:
Koepka won the PGA Championship this season, becoming the first man to defend his title since Tiger Woods did so in 2007. He also won the WGC-FedEx St. Jude Invitational and C.J. Cup, and per Golf Digest's Joel Beall, he was awarded the PGA of America Player of the Year.
Fowler won the Waste Management Phoenix Open this year, his fifth title on the PGA Tour.
Woods will captain this year's American team at the Presidents Cup, which will start at the Royal Melbourne Golf Club in Australia on December 12. The venue is the place where the International Team has taken its only Presidents Cup win, in 1998.
The American team will feature the likes of Dustin Johnson, Justin Thomas, Xander Shauffele and Bryson DeChambeau. The International team is headlined by Hideki Matsuyama, Louis Oosthuizen and Jason Day.