Baez is batting second and playing second base in the series opener against the Rockies. He was the ninth overall pick in the 2011 draft and was the organization's player of the year last season after hitting 37 home runs in Class A and Double-A.
Baez had his contract selected from Triple-A Iowa on Tuesday, and right-hander Blake Parker was optioned to Chicago's top minor league affiliate.
Baez got off to a slow start this year in the minors, but has been playing well of late. The middle infielder, who is known for his tremendous bat speed, hit .300 with 12 homers and 34 RBIs in his last 32 games for Triple-A Iowa.