The Royals acquired McArthur from the Phillies on Monday in exchange for outfielder Junior Marin and cash considerations.
Though McArthur will most likely settle into the rotation at Triple-A Omaha with his new organization, he'll join the Royals' 40-man roster with Kansas City designating outfielder/designated hitter Franmil Reyes for assignment in a corresponding move. McArthur, who has yet to reach the majors, has logged a 7.31 ERA, 1.75 WHIP and 15:7 K:BB across 16 innings for Triple-A Lehigh Valley this season.