ESPN's Jeff Passan reported right-handed pitcher Kasey Kalich will go back to Kansas City in the deal.
The 29-year-old is batting .192 with 13 home runs and will look to turn things around in Atlanta.
Kansas City has fallen to 45-56 as the American League Central division race tightened between the Chicago White Sox and Cleveland. That left the rebuilding Royals with little to play for this year, but moving Soler—and his expiring one-year, $8.05 million contract—could pay dividends in the near future.
The Royals' farm system is looking like one of the best in baseball yet again thanks to prospects such as shortstop Bobby Witt Jr., left-handers Daniel Lynch and Asa Lacy along with righty Jackson Kowar.
That might not be much consolation as another year in the careers of Salvador Perez, Whit Merrifield, Andrew Benintendi and Carlos Santana is wasted on a losing season. Still, moving Soler was always a likely scenario given his pending free agency.
The 2019 AL home run leader and 2016 World Series champion has plenty to offer a contender down the stretch.
Now he'll get the opportunity to prove it. Entering Friday, the Braves are four games back of the New York Mets in the National League East.