The Cubs entered Sunday 4-for-40 with runners in scoring position through their first five games.
"It's good, because we haven't scored any runs," Castro said of the switch. "That's why I'm going to try to do something to get some more runs. It's good. I feel good. That's a RBI position. Maybe we'll have men on base in scoring position."
Renteria also wanted to take advantage of Castro's numbers against Phillies starter A.J. Burnett. The Cubs shortstop is now 10-for-25 in his career against the right-hander.
The Cubs manager didn't talk to Castro before posting the lineup.
"He's not a guy I worry about," Renteria said.
Castro understood the switch.
"We haven't scored runs," Castro said. "I take [batting sixth] in a good way. We've left a lot of people in scoring position. I know Ricky is trying to do something."