Castillo, who will wear uniform No. 53, was at Wrigley Field on Wednesday for the Cubs' game against the Cardinals. Catcher Koyie Hill started Wednesday, but Castillo may get the start Thursday when starting pitcher Casey Coleman starts the series against the Cardinals.
Soto underwent an MRI on Wednesday after he injured his groin during the top of the first inning Tuesday. He was batting .226 with eight doubles, three home runs and 12 RBIs in 31 games.
Castillo, 24, made his Major League debut with the Cubs last season and batted .300 with four doubles, one home run and five RBIs in seven games. He spent nearly all of 2011 Spring Training with the Cubs and batted .632 (12-for-19) with an .842 slugging percentage in 17 Cactus League games.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.