Castro has been scratched from the lineup for Saturday's game against the Diamondbacks due to low-back tightness.
TJ Rumfield will now start at first base with Sterlin Thompson at DH, opening up right field for Tyler Freeman against right-hander Zac Gallen. While the severity of Castro's injury is unknown, it's poor timing for the 29-year-old, as he was showing signs of life with eight hits in his last 21 at-bats.