Castro has been working out with a physical therapist in Houston, and manager Brad Mills said the catcher had his best day of rehab yet on Monday. Castro said he was set to meet with the Astros to map out his possible return.
"They're really happy with way it's responding," he said.
Castro remains unsure of a timetable and whether he'll have to get a few at-bats on a Minor League rehab assignment.
"I'm just happy with how it's responded so far and it definitely feels better than the first time I had it drained," he said. "I'm really happy about that. We look forward to going from here and hope it stays that way and there will be no problems the rest of the year."
Meanwhile, shortstop Jed Lowrie, who is expected to miss at least another month with a sprained right ankle and injury to his peroneal nerve, is still wearing a protective brace and using crutches to walk. He's also undergoing physical therapy.
"I've been doing some real simple stuff, trying to at least stay away from the knee and keep my hips moving and make sure nothing shuts down in that area," Lowrie said.