Soto is batting .226 with eight doubles, three home runs and 12 RBI in 31 games with the Cubs this season.
Castillo made his major league debut with the Cubs last season, batting .300 with a homer and five RBIs in seven games.
Castillo was hitting a combined .242 between Iowa and Single-A Daytona. He also had five doubles, three home runs and 11 RBIs in 26 games.
Information from The Associated Press was used in this report.