Mills was hoping that Castro, who missed all of last season following ACL surgery on his knee, could catch three innings and work his way up to nine innings before being activated. Corpus Christi opens a four-game series Friday in Midland.
"That has not totally been set in stone yet, but that's kind of what we had talked about," Mills said. "If it's not Friday [he begins his rehab], it's going to be fairly soon anyway, to get him out and get him going again."
Castro is hitting .254 with two homers and 20 RBIs this year, but was batting .301 with a .380 on-base percentage in his 83 at-bats prior to his knee injury.
Meanwhile, Mills said shortstop Jed Lowrie -- on the disabled list since July 15 with a sprained right ankle and damage to his nerve -- has begun wearing another leg brace that will allow him to have more range of motion in the knee.