Castro has been on the 15-day disabled list with a right knee effusion since July 8.
He has played in two rehab games with Corpus Christi. He played four innings and went 1-for-2 with a double Saturday. Mills said Castro blocked "a couple balls," which was an encouraging sign.
"He felt good. That's the biggest thing," Mills said. "Swelling and stuff was not an issue, so that was absolutely huge to be able to do that."
Castro was hitting .254 with a .330 on-base percentage before the injury.