Rosenthal, 29, signed a minor-league deal in January that included an invite to spring training with the Royals. He allowed zero runs on three hits while striking out nine over five innings pitched.
The former All-Star owned a 3.01 ERA while notching 121 saves from 2013 to 2017 as a member of the St. Louis Cardinals. The flamethrower missed the 2018 campaign after requiring Tommy John surgery.
Rosenthal is coming off a forgettable season split between the Washington Nationals and Detroit Tigers. He authored a 22.74 ERA over 6 1/3 innings with the former, memorably tying the record for consecutive appearances without recording an out before he was released. Rosenthal then posted a 7.00 ERA over nine frames with the Tigers.
In a corresponding move, the Royals designated left-handed starter Eric Skoglund for assignment. The 27-year-old owns a 6.61 ERA in 27 games (22 starts) over three seasons with Kansas City.