Castro ended the Cubs' stretch of 17 scoreless innings when he delivered a go-ahead RBI single with one out in the seventh to lift Chicago to a 2-0 victory over the Cardinals.
Castro had followed Arrieta's lead. Arrieta's message to Castro was: "I got your back." When Arrieta exited after the seventh, Castro went up to him.
"I told him the same thing, 'We got your back now, we got your back,'" Castro said, "and we scored two runs that inning and won the game."
Arrieta was impressed by the shortstop's honesty and maturity.
"Those kind of things might not have happened last year," Arrieta said.
Castro was not happy with himself after the Cardinals' Lance Lynn struck him out in the fourth.
"I got mad because I wanted to get that run," Castro said. "We haven't been good with runners in scoring position. I still have my confidence and got the job done."
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.