The Royals selected Bummer's contract from Triple-A Omaha on Sunday.
The left-hander joined Kansas City on a minor-league deal July 20 and will get a look in the majors after delivering four scoreless outings for Omaha. Bummer made 19 appearances for Atlanta earlier this year and surrendered 13 earned runs with a 13:10 K:BB over 15.1 innings.