Zobrist, batting .268 with six homers and 31 RBIs, was batting sixth against Toronto right-hander Marco Estrada.
Manager Ned Yost said the versatile Zobrist will be used primarily in left field but will also play right and second base.
Outfielder Paulo Orlando was optioned to Triple-A Omaha to make room for Zobrist, who was acquired from Oakland on Tuesday.
Yost said the Royals decided to demote Orlando and keep third baseman Cheslor Cuthbert on the roster because Cuthbert has played well at third base, a position Zobrist has not played frequently. Cuthbert also hits right-handed, and the Royals are due to face multiple left-handed starters in the next few days.
First base coach Rusty Kuntz gave up jersey number 18 to Zobrist. Kuntz will wear 81.